Stomach has small sudden rush (4)
I believe the answer is:
gust
'sudden rush' is the definition.
(I've seen this in another clue)
'stomach has small' is the wordplay.
'stomach' becomes 'gut' (I've seen this before).
'has' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'gut' placed around 's' is 'GUST'.
(Other definitions for gust that I've seen before include "Puff (of wind)" , "Blast of air" , "A big blow" , "Sudden rush of wind" , "Squall" .)