Firm American lawyer closing bars (4)
I believe the answer is:
coda
'closing bars' is the definition.
(I've seen this before)
'firm american lawyer' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'american lawyer' becomes 'DA' (abbreviation for District Attorney).
'co'+'da'='CODA'
(Other definitions for coda that I've seen before include "Tailpiece" , "Novel ending" , "End piece written for a musical work" , "Conclusion of work" , "tail" .)