Fit in criminal arrest record (7)
I believe the answer is:
conform
'fit in' is the definition.
(thesaurus)
'criminal arrest record' is the wordplay.
I cannot quite see how this works, but
'criminal' could be 'con' (abbreviation for 'convict') and 'con' is present in the answer.
'record' could be 'form' (I've seen this before) and 'form' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for conform that I've seen before include "Agree" , "Keep to rules or general custom" , "Fall into line" , "Follow the herd" , "become fitter?" .)