Stingers used to be an afterthought (5)
I believe the answer is:
wasps
'stingers' is the definition.
(I've seen this in another clue)
'used to be an afterthought' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'an afterthought' becomes 'ps' (ps stands for postscript).
'was'+'ps'='WASPS'
(Other definitions for wasps that I've seen before include "English Protestant stingers?" , "Insects; rugby union team" , "Ones playing together" , "Members of rugby team" , "Stinging hymenopterans" .)