I predict start of rain after a month, mostly (5)
I believe the answer is:
augur
'i predict' is the definition.
'start of rain after a month mostly' is the wordplay.
'start of rain' becomes 'gur' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'after' means one lot of letters go next to another.
'a month' becomes 'aug' (Aug is an example).
'mostly' means to remove the last letter.
'aug' with its last letter taken off is 'au'.
'gur' put after 'au' is 'AUGUR'.
(Other definitions for augur that I've seen before include "Portent" , "Foretell from signs" , "Reader of omens" , "Portend a good or/bad outcome" , "Portend; Roman official" .)