Flower retired TV actor is wearing (6)
I believe the answer is:
thames
'flower' is the definition.
(River Thames, something which flows)
'retired tv actor is wearing' is the wordplay.
'retired' shows that the letters should be reversed in order.
'tv' becomes 'set' (television set).
'actor' becomes 'ham' (ham is a kind of actor).
'is wearing' is an insertion indicator.
'set' written backwards gives 'tes'.
'tes' enclosing 'ham' is 'THAMES'.
(Other definitions for thames that I've seen before include "'Old Father ___'" , "River at Windsor" , "London's river (6)" , "The Isis at Oxford" , "Capital's river" .)