How much is swallowed at one time? That's hard to say (8)
I believe the answer is:
mouthful
'how much is swallowed at one time?' is the definition.
I can't tell whether this defines the answer.
'hard to say' is the wordplay.
'hard' becomes 'ful' (I can't explain this - if you can you should believe this answer much more).
'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' after 'mouth' is 'MOUTHFUL'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for mouthful that I've seen before include "Something difficult to say or understand" , "Complicated word or phrase" , "Difficult word or phrase to pronounce" , "Outburst of strong language" .)