Defendant keeping male cat as familiar (10)
I believe the answer is:
accustomed
'familiar' is the definition.
(I've seen this before)
'defendant keeping male cat' is the wordplay.
'defendant' becomes 'accused' (synonyms).
'keeping' is an insertion indicator (some letters kept inside).
'male cat' becomes 'tom' (I've seen this before).
'accused' placed around 'tom' is 'ACCUSTOMED'.
'as' acts as a link.
(Other definitions for accustomed that I've seen before include "Used to" , "familiar" , "usual" .)