A young animal backed into insect (4)
I believe the answer is:
pupa
'insect' is the definition.
(pupa is a kind of insect)
'a young animal backed' is the wordplay.
'young animal' becomes 'pup' (I've seen this before**).
'backed' says the letters should be written backwards.
'a'+'pup'='apup'
'apup' in reverse letter order is 'PUPA'.
'into' acts as a link.
(Other definitions for pupa that I've seen before include "A chrysalis as of butterfly" , "creepy-crawly" , "not yet adult" , "appearance of immaturity" , "developing creature" .)