Despite the fact that it's chewy and hard to swallow (6)
I believe the answer is:
though
'despite the fact that' is the definition.
(synonyms)
'chewy and hard to swallow' is the wordplay.
I cannot quite see how this works, but
'hard' could be 'h' (abbreviation used in pencil classifications) and 'h' is found in the answer.
'chewy' could be 'tough' (similar in meaning) and 'tough' is found in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for though that I've seen before include "However" , "Even if" , "Despite the fact that" , "Nevertheless" , "Concessive conjuction" .)