Scouse is different across river -- that's sweet! (7)
I believe the answer is:
sucrose
'that's sweet' is the definition.
'scouse is different across river' is the wordplay.
'is different' indicates an anagram.
'across' indicates putting letters inside.
'river' becomes 'r'.
'scouse' with letters rearranged gives 'sucose'.
'sucose' placed around 'r' is 'SUCROSE'.
(Other definitions for sucrose that I've seen before include "Something in nectar" , "Sweetening agent -- Croesus (anag)" , "Sweet substance" , "Sugar's required" , "Sweet compound" .)