Well-dressed US lawyer quietly sent back negotiator (6)
I believe the answer is:
dapper
'well-dressed' is the definition.
(I've seen this before)
'us lawyer quietly sent back negotiator' is the wordplay.
'us lawyer' becomes 'DA' (abbreviation for District Attorney).
'quietly' becomes 'pp' (musical abbreviation for pianissimo).
'sent back' says the letters should be written backwards.
'negotiator' becomes 're' (I am not sure about this - if you are sure you should believe this answer much more).
're' back-to-front is 'er'.
'da'+'pp'+'er'='DAPPER'
Can you help me to learn more?
(Other definitions for dapper that I've seen before include "Neat and well dressed" , "Natty, dashing" , "angler letting bait bob on water?" , "bouncer" , "Smart - well presented" .)