Male animal parts defendant used (10)
I believe the answer is:
accustomed
'used' is the definition.
(I've seen this before)
'male animal parts defendant' is the wordplay.
'male animal' becomes 'tom' (male cat).
'parts' indicates putting letters inside (inserted letters separate or part others).
'defendant' becomes 'accused' (synonyms).
'tom' inserted into 'accused' is 'ACCUSTOMED'.
(Other definitions for accustomed that I've seen before include "usual" , "Used to" , "familiar" .)