A second source found, too (2,4)
I believe the answer is:
as well
'too' is the definition.
('as well' can be a synonym of 'too')
'a second source' is the wordplay.
'a second' becomes 'as' (I've seen this in other clues).
'source' becomes 'well' (well is a kind of source**).
'as'+'well'='AS WELL'
'found' acts as a link.
(Other definitions for as well that I've seen before include "Also, additionally" , "To boot" , "on top" , "Appropriate" , "A swell sort of addition" .)