British comedienne dropping very old sitcom actor (6)
I believe the answer is:
briers
'old sitcom actor' is the definition.
(The Good Life actor Richard Briers)
'british comedienne dropping very' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'comedienne' becomes 'Rivers' (Joan Rivers).
'dropping' indicates named letters should be taken away.
'very' becomes 'v' (abbreviation).
'rivers' with 'v' taken out is 'riers'.
'b'+'riers'='BRIERS'
(Other definitions for briers that I've seen before include "Good (the actor)" , "Thorny bushes like wild roses" .)