Stripy insect used to be pest at first (4)
I believe the answer is:
wasp
'stripy insect' is the definition.
(I know that wasps are stripy)
'used to be pest at first' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'at first' suggests taking the first letters.
The first letter of 'pest' is 'p'.
'was'+'p'='WASP'
(Other definitions for wasp that I've seen before include "Being in the American elite" , "Insect often irritating in summer" , "Flyer" , "Insect; thin (waist)" , "A hymenopteran" .)