Aggressive when it’s back in the burrow (7)
I believe the answer is:
'aggressive' is the definition.
(similar in meaning)
'it's back in the burrow' is the wordplay.
'back' shows that the letters should be reversed in order.
'in the' is an insertion indicator.
'burrow' becomes 'hole' (a hole an animal lives in).
'its' backwards is 'sti'.
'sti' put within 'hole' is 'HOSTILE'.
'when' acts as a link.
(Other definitions for hostile that I've seen before include "Inimical" , "harsh" , "Enemy" , "The soil strangely is unfriendly" , "Bellicose" .)