Grow small source of water (5)
I believe the answer is:
'grow' is the definition.
(swelling is a kind of growing)
'small source of water' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'source of water' becomes 'well' (I've seen this before).
(Other definitions for swell that I've seen before include "Excellent! - become larger" , "marvellous" , "Balloon - dandy" , "Grow larger; excellent" , "Become larger like toff" .)