Sounds like rain as a rule (5)
I believe the answer is:
reign
'as a rule' is the definition.
(I've seen this before)
'sounds like rain' is the wordplay.
'sounds like' indicates a 'sounds like' (homophone) clue.
'rain' sounds like 'REIGN'.
(Other definitions for reign that I've seen before include "Period on the throne" , "Niger (anag)" , "Monarch's time in the job" , "Sit on a throne" , "period in power" .)