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' says the letters should be written in reverse.
'going round' indicates putting letters inside (some letters go around others).
'bad actor' becomes 'ham' (a ham is someone who overacts).
'set' back-to-front is 'tes'.
'tes' enclosing '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" .)