Dog nibbled companion (5)
I believe the answer is:
bitch
'dog' is the definition.
(I've seen this before)
'nibbled companion' is the wordplay.
'nibbled' becomes 'bit' (I've seen this in other clues).
'companion' becomes 'ch' (I've seen this in other clues).
'bit'+'ch'='BITCH'
(Other definitions for bitch that I've seen before include "Female dog" , "grouse" , "Retriever likely to drop" , "Female animal" , "Malicious woman" .)