Surfeit eaten but not swallowed is a problem for some diners (6)
I believe the answer is:
gluten
'a problem for some diners' is the definition.
(some people are gluten intolerant)
'surfeit eaten but not swallowed' is the wordplay.
'surfeit' becomes 'glut' (synonyms).
'but not' suggests deleting specific letters.
'swallowed' becomes 'ate' (swallow can mean to eat).
'eaten' with 'ate' removed is 'en'.
'glut'+'en'='GLUTEN'
'is' acts as a link.
(Other definitions for gluten that I've seen before include "Protein source" , "Protein in wheat grains" , "Substance in cereals" , "Substance in cereal grain" , "Protein substance from cereal grains that gives elasticity to dough" .)