Canary egg brought into school (6)
I believe the answer is:
snitch
'canary' is the definition.
(thesaurus)
'egg brought into school' is the wordplay.
'egg' becomes 'nit' (a nit is an insect egg).
'brought into' means one lot of letters goes inside another.
'school' becomes 'sch' (abbreviation).
'nit' inserted within 'sch' is 'SNITCH'.
(Other definitions for snitch that I've seen before include "Whistleblower (informal)" , "Grass" , "Telltale (sl.)" , "Inform on, tell tales" , "Police informer" .)