Small spring to grow (5)
I believe the answer is:
swell
'grow' is the definition.
(I know that swell is a more specific form of grow)
'small spring' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'spring' becomes 'well' (both can mean a source of water).
's'+'well'='SWELL'
'to' acts as a link.
(Other definitions for swell that I've seen before include "Informally good, grow larger" , "Bulge; a toff" , "Splendid - organ stop" , "part of organ" , "organ feature" .)