Warning powerful rainstorm's beginning to come in (7)
I believe the answer is:
portent
'warning' is the definition.
(I've seen this before)
'powerful rainstorm's beginning to come in' is the wordplay.
'powerful' becomes 'potent' (similar in meaning).
'beginning to' indicates taking the first letters.
'come in' means one lot of letters goes inside another.
The first letter of 'rainstorms' is 'r'.
'potent' placed around 'r' is 'PORTENT'.
(Other definitions for portent that I've seen before include "Sign of thing to come" , "Sign or warning of an event likely to happen" , "Augury" , "Foreboding" , "Sign, omen" .)