Healthy small sources of water (5)
I believe the answer is:
wells
'sources of water' is the definition.
(I've seen this before)
'healthy small' is the wordplay.
'healthy' becomes 'well' ('well' can be similar in meaning to 'healthy').
'small' becomes 's' (abbreviation - e.g. clothes size).
'well'+'s'='WELLS'
(Other definitions for wells that I've seen before include "Cathedral city; sci-fi writer" , "What a boring person might produce" , "H. G. . . . . ., science-fiction writer" , "Springs" , "Oil sources" .)