Back to arrest ambassador? Try again (6)
I believe the answer is:
rehear
'try again' is the definition.
('hear' can be a synonym for 'try' so perhaps 'rehear' is OK for 'try again')
'back to arrest ambassador?' is the wordplay.
'back' becomes 'rear' ('rear' can be a synonym of 'back').
'to arrest' means one lot of letters goes inside another.
'ambassador?' becomes 'he' (His or Her Excellency).
'rear' placed around 'he' is 'REHEAR'.
(Other definitions for rehear that I've seen before include "Hold the trial again." , "Retry (a lawsuit)" , "Try again (in court)" , "be a sympathetic listener, not for the first time" .)