River set back, going round bad actor (6)
I believe the answer is:
thames
'river' is the definition.
(English river)
'set back going round bad actor' is the wordplay.
'back' shows that the letters should be reversed in order.
'going round' means one lot of letters goes inside another (some letters go around others).
'bad actor' becomes 'ham' (a ham is someone who overacts).
'set' back-to-front is 'tes'.
'tes' placed around 'ham' is 'THAMES'.
(Other definitions for thames that I've seen before include "London river" , "The Ma's in the river" , "It flows through London" , "River at Henley" , "London flood control" .)