Such insects used to be an afterthought (5)
I believe the answer is:
wasps
'such insects' is the definition.
'wasps' can be an answer for 'insects' (wasp is a kind of insect). I'm unsure of the 'such' bit.
'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'
Can you help me to learn more?
(Other definitions for wasps that I've seen before include "Stinging hymenopterans" , "Ones playing together" , "Striped stinging insects" , "Insects; rugby union team" , "Members of rugby team" .)