Corn — leave the outer part, it’s tough to chew (7)
I believe the answer is:
gristle
'part it's tough to chew' is the definition.
I can't judge whether this defines the answer.
'corn leave the outer' is the wordplay.
'corn' becomes 'grist' (I've seen this before).
'the outer' suggests removing the centre.
'leave' with its centre taken out is 'le'.
'grist'+'le'='GRISTLE'
Can you help me to learn more?
(Other definitions for gristle that I've seen before include "Chewy cartilage" , "Stir leg (anag.)" , "Cartilage in meat" , "Glister with animal tissue" , "Stir leg to get tough tissue in meat" .)