Cat kept by defendant as familiar (10)
I believe the answer is:
accustomed
'familiar' is the definition.
(thesaurus)
'cat kept by defendant' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'kept by' means one lot of letters goes inside another.
'defendant' becomes 'accused' ('accused' can be a synonym of 'defendant').
'tom' placed into 'accused' is 'ACCUSTOMED'.
'as' is the link.
(Other definitions for accustomed that I've seen before include "Used to" , "familiar" , "usual" .)