Coach let off activity (6)
I believe the answer is:
bustle
'activity' is the definition.
(I've seen this before)
'coach let off' is the wordplay.
'coach' becomes 'bus' (synonyms).
'off' is an anagram indicator.
'let' anagrammed gives 'tle'.
'bus'+'tle'='BUSTLE'
(Other definitions for bustle that I've seen before include "Stir of activity" , "Excited activity of subtle sort" , "Hurried activity - old padded support for woman's dress" , "Frame under a dress" , "Excited activity - or padding at rear of old skirt" .)