Unexpected blow son received in belly (4)
I believe the answer is:
gust
'unexpected blow' is the definition.
(I've seen this in another clue)
'son received in belly' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'received in' indicates putting letters inside.
'belly' becomes 'gut' (informal term for the stomach or gut).
's' put into 'gut' is 'GUST'.
(Other definitions for gust that I've seen before include "A strong current of air" , "Puff (of wind)" , "Brief rush of wind" , "Squall" , "14 [OUTBURST]" .)