A second spring besides (2,4)

I believe the answer is:

as well

'besides' is the definition.
(thesaurus)

'a second spring' is the wordplay.
'second' becomes 's' (common abbreviation).
'spring' becomes 'well' (both can mean a source of water).
'a'+'s'+'well'='AS WELL'

(Other definitions for as well that I've seen before include "To boot" , "Appropriate" , "on top" , "with equal effect" , "A swell sort of addition" .)

I've seen this clue in The Everyman puzzle in The Observer.

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