Strong and sour etc. after modifying sweet stuff (8)
I believe the answer is:
fructose
'sweet stuff' is the definition.
(type of sugar)
'strong and sour etc after modifying' is the wordplay.
'strong' becomes 'f' (musical symbol for 'forte', 'strong' in Italian).
'and' means one lot of letters go next to another.
'after' means one lot of letters go next to another.
'modifying' is an anagram indicator (I've seen 'modified' mean this).
'sour'+'etc'='souretc'
'souretc' with letters rearranged gives 'ructose'.
'f'+'ructose'='FRUCTOSE'
(Other definitions for fructose that I've seen before include "A simple sugar, in honey for instance" , "It's sweet" , "Sweetener" , "Kind of sugar" , "Fruit-sugar" .)