Spoonerisms are similar to letter exchanges, but use the the Reverend Spooner's well known problem with words. If you don't know already, he was forever getting his mucking words fuddled.
Here's an example:
To get from TIE PIN to PIE TIN the setter uses a Spoonerism indicator:
Tie pin, according to Spooner ...
Spooner's tie pin ...
Any phrase that mentions Spooner is likely to be a Spoonerism indicator. When you see one, look to exchange the first letter or letters (or the first syllables) of the synonyms of the fodder words.
NOTE: I am not very good at writing Spooner clues - in fact, in the thousands of clues I've written, I only have two Spooner's in my database. Here are both and both are dodgy - but hopefully they'll give you an idea of how the clue type works.
First-class bit of advice from Spooner (6)
first-class - TIPTOP
bit of advice - TOP TIP
from Spooner - Spooner indicator
Here's me other one:
Spooner's way to secure a kipper for a baker? (3,3)
spooner's - spooner indicator
way to secure - PIN
a kipper - TIE (type of tie)
for - link
a baker? - PIE TIN (something used for baking)
NOTE: Told you I was rubbish at them - I'll be trying to think some up to provide a more accurate example ... but don't hold your breath.