The pure cryptic definition (or CD) is different from most cryptic clue types because it does not have a subsidiary indication. It functions by defining an answer word accurately in its cryptic reading, but in a way that seeks to send the solver to a completely different answer with its surface reading.
Here is an example:
Food parcel cracked open by soldiers? (8)
This pure cryptic definition (the whole clue is the definition with no subsidiary wordplay) seeks to make the solver think the answer is something to do with front line soldiers and the food parcel they receive from family or friends.
However, the answer is EGGSHELL and when one reads the clue as a FOOD PARCEL meaning something containing food, CRACKED OPEN as a hint to an eggshell being opened by a spoon, BY meaning 'close to', and SOLDIERS as little strips of toast one has with a boiled egg, the real answer becomes be clear.
In which crates are stacked-up by the tower? (7,7)
Here the surface attempts to make the solver think of some sort of warehouse where crates are stored in stacks. However, the answer is HOLDING PATTERN, and one can see this with the mention of CRATES (a nickname of aircraft) being STACKED-UP (a stack is a term for planes awaiting landing) and, of course, TOWER (the control tower of an airport).
NOTE: A cryptic definition may carry a question mark to help indicate that lateral thinking is required, but it will not always do so.
Cryptic Definitions with Worplay
Cryptic definitions are also used in conjunction with wordplay to give the more standard two ways to get to the answer word.
Here are is an example:
Have a go at father for being bad at multiplication? (5)
have a go at - PAN
father - DA
for - definition link
being bad at multiplication? - cryptic definition (being as a noun)
Note that the definition seems to indicate the father being bad at maths, but is actually a definition of an animal (being as a noun) that's notoriously bad at reproduction.
To show the difference between a part cryptic definition and a straight definition I include the following example. Note that the definition is not really cryptic, but only appears that way as it seeks to fool the solver by the association with the first part of the clue:
Tenor gets garment in small - it may give a bad fit (6,5)
tenor - T (abbreviation for tenor)
gets - positional indicator
garment - ROBE
in - insertion indicator
small - SLIGHT
it may give a bad fit - cryptic definition
Note that the definition needs the rest of the clue to deceive the solver into thinking of a fat tenor having a bad clothing fit after buying a small garment. Without the rest of the clue there is no real deception, just a (rather weak) definition indicating the dangerous effect a strobe light can have on certain people.