Oil producer’s beer cask knocked over by clown (7)
I believe the answer is:
coconut
'oil producer's' is the definition.
(as in coconut oil)
'beer cask knocked over by clown' is the wordplay.
'beer cask' becomes 'tun' (I've seen this before).
'knocked over' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'clown' becomes 'coco' (I've seen this before).
'tun' written backwards gives 'nut'.
'nut' after 'coco' is 'COCONUT'.
(Other definitions for coconut that I've seen before include "Pina colada ingredient" , "fair target" , "Hard thing" , "Shy target?" , "Large edible fruit" .)