Rain cats and dogs (4)
I believe the answer is:
pour
'rain cats and dogs' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for pour that I've seen before include "Serve liquid" , "Cause to flow as from pot to cup" , "Cause to flow in a stream" , "to drop, when it's heavy" , "(Of rain) fall heavily" .)