Quiet coach from the south expected (6)
I believe the answer is:
subdue
'quiet' is the definition.
(thesaurus)
'coach from the south expected' is the wordplay.
'coach' becomes 'bus' (synonyms).
'from the south' is a reversal indicator (in a down clue, letters go up).
'expected' becomes 'due' (similar in meaning).
'bus' reversed gives 'sub'.
'sub'+'due'='SUBDUE'
(Other definitions for subdue that I've seen before include "Overpower and bring under control" , "Moderate" , "Overcome or inhibit" , "Overcome, bring under control" , "Quash" .)