Contents of case — source of water too (2,4)
I believe the answer is:
as well
'too' is the definition.
(synonyms)
'contents of case source of water' is the wordplay.
'contents of' means to look at the middle letters (the letters on the inside of the word).
'source of water' becomes 'well' (water may come from a well).
The centre of 'case' is 'as'.
'as'+'well'='AS WELL'
(Other definitions for as well that I've seen before include "with equal effect" , "on top" , "Appropriate" , "A swell sort of addition" , "Also, additionally" .)