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 defines the answer.
'hard to say' is the wordplay.
'hard' becomes 'ful' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' means one lot of letters go next to another (I've seen this in other clues).
'say' becomes 'mouth' (I've seen this in other clues).
'ful' 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" .)