Headless maggot snaffled by young canary (9)
I believe the answer is:
informant
'headless maggot' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'snaffled by young canary' is the wordplay.
'snaffled by' means one lot of letters goes inside another (snaffle means to take or steal).
'young' becomes 'infant' (similar in meaning).
'canary' becomes 'orm' (I can't justify this - if you can you should believe this answer much more).
'infant' going around 'orm' is 'INFORMANT'.
Can you help me to learn more?
(Other definitions for informant that I've seen before include "One who sings" , "Rat" , "source of dope" , "Sneak" , "Telltale" .)