Odd, Even & Alternate Clues
The odd & even clue type (also known as alternate) makes part or all of the answer word by selecting letters from words at regular intervals. An odd/even clue's fodder (the word[s] in which the letters are taken from) is unlikely to need the solver to find a synonym first.
You can spot this clue type by looking for indicators that suggest the fodder be taken regularly or oddly and, if we are very quiet, we may just be able to observe some now ...
Here are a couple of examples:
For the setter to select the word TEE from THEME they may write something such as:
theme oddly ...
... here ODDLY is the alternate letter indicator and tells the solver to select only the odd letters of ThEmE to create TEE.
The same came be done with an alternate letter subtraction. If a setter wants to indicate ME from AMBER they may write:
ignoring the odds, Amber ...
... here the solver is being told to 'ignore the odd letters' in the fodder. And so aMbEr becomes ME.
Any word or phrase that fairly indicates alternate letters or a sequence of letters are to be selected or rejected may be used as an indicator. Here a some more examples:
Alternate letter indicators: REGULARLY/OFF AND ON
Pure odd letter clue:
Go brown in train, oddly (3)
go brown - definition
in - link (definition found in wordplay)
train - fodder
oddly - odd letter selection indicator
Part odd/even clue:
Drunk adult regularly carried by orderly (5)
drunk - definition
adult - fodder
regularly - every other letter indicator
carried by - containment indicator
orderly - tidy
Regular interval clue:
Call of "lower!" made regularly in game show spot (3)
call of lower - MOO (definition)
made regularly in - regular sequence indicator
game show spot - fodder
Note that the fodder is taken every third letter. Clues that don't specifically indicate how many letters there are in the sequence are likely to be shorter to retain their fairness.