Wet weather is the rule, it's said (4)
I believe the answer is:
rain
'wet weather' is the definition.
(I've seen this before)
'the rule it's said' is the wordplay.
'the rule' becomes 'reign' (reign is a kind of rule).
'it's said' shows a homophone (sound like).
'reign' sounds like 'RAIN'.
'is' acts as a link.
(Other definitions for rain that I've seen before include "Precipitation" , "Storm" , "Pelt" , "See 12" , "Wet weather" .)