Use mortars in wet weather (9)
I believe the answer is:
rainstorm
'wet weather' is the definition.
'rainstorm' can be an answer for 'weather' (I have seen 'bad weather' mean 'rainstorm' so perhaps 'weather' could also mean 'rainstorm'). I'm unsure of the 'wet' bit.
'use mortars in' is the wordplay.
'use' is an anagram indicator.
'mortars'+'in'='mortarsin'
'mortarsin' anagrammed gives 'RAINSTORM'.
Can you help me to learn more?
(Other definitions for rainstorm that I've seen before include "bad weather" , "squall" , "Heavy fall" , "Meteorological phenomenon" , "In bad fall" .)