Bucket down, causing hunt assembly to retreat (4)
I believe the answer is:
teem
'bucket down' is the definition.
(I've seen this before)
'hunt assembly to retreat' is the wordplay.
'hunt assembly' becomes 'meet' (a party meeting to go hunting).
'to retreat' says the letters should be written in reverse.
'meet' written backwards gives 'TEEM'.
'causing' acts as a link.
(Other definitions for teem that I've seen before include "(Of rain) pour down" , "Be full" , "Rain buckets" , "Be prolific" , "stream" .)