Crossword Genius

Singular source for expansion (5)

Ross

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" .)

I've seen this clue in The Telegraph.
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