Insect's victim agonised to some extent (5)
I believe the answer is:
imago
'insect's' is the definition.
(imago is a kind of insect)
'victim agonised to some extent' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'IMAGO' can be found hidden inside 'victim agonised'.
(Other definitions for imago that I've seen before include "Insect in its adult form" , "in the developmental stage" , "That's as far as a butterfly can go" , "Adult, post-metamorphosis insect" , "Maybe comma" .)