Stinger used to be head prefect (4)
I believe the answer is:
wasp
'stinger' is the definition.
(insect with a sting)
'used to be head prefect' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'head' suggests taking the first letters.
The first letter of 'prefect' is 'p'.
'was'+'p'='WASP'
(Other definitions for wasp that I've seen before include "spiteful person?" , "Sort of privileged American" , "Insect often irritating in summer" , "A hymenopteran" , "Insect with a formidable sting" .)