A source of water includes sulphur too (2,4)
I believe the answer is:
as well
'too' is the definition.
(synonyms)
'a source of water includes sulphur' is the wordplay.
'source of water' becomes 'well' (water may come from a well).
'includes' indicates putting letters inside.
'sulphur' becomes 'S' (synonyms).
'a'+'well'='awell'
'awell' enclosing 's' is 'AS WELL'.
(Other definitions for as well that I've seen before include "with equal effect" , "To boot" , "A swell sort of addition" , "Appropriate" , "Also, additionally" .)