How much food is on the fork is hard to say (8)
I believe the answer is:
mouthful
'how much food is on the fork' is the definition.
I can't tell whether this definition defines the answer.
'hard to say' is the wordplay.
'hard' becomes 'ful' (I can't explain this - if you can you should give a lot more credence to this answer).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'say' becomes 'mouth' (I've seen this in other clues).
'ful' put after 'mouth' is 'MOUTHFUL'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for mouthful that I've seen before include "Difficult word or phrase to pronounce" , "Something difficult to say or understand" , "Outburst of strong language" , "Complicated word or phrase" .)