Singular source for expansion (5)
I believe the answer is:
swell
'expansion' is the definition.
(thesaurus)
'singular source' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'source' becomes 'well' (well is a kind of source).
's'+'well'='SWELL'
'for' acts as a link.
(Other definitions for swell that I've seen before include "Plump" , "organ feature" , "VIP?" , "Bulge; a toff" , "Distend" .)