Bad weather expected to occur mostly in spring month, returning (9)
I believe the answer is:
rainstorm
'bad weather' is the definition.
(I've seen this before)
'expected to occur mostly in spring month returning' is the wordplay.
'expected to occur' becomes 'instore' (in store).
'mostly' means to remove the last letter.
'in' is an insertion indicator.
'spring month' becomes 'mar' (March).
'returning' says the letters should be written backwards.
'mar' back-to-front is 'ram'.
'instore' with its final letter removed is 'instor'.
'instor' put within 'ram' is 'RAINSTORM'.
(Other definitions for rainstorm that I've seen before include "power shower?" , "Meteorological phenomenon" , "squall" , "Heavy downpour" , "In bad fall" .)