Working at a diet's getting stuffed (8)
I believe the answer is:
satiated
'stuffed' is the definition.
('satiate' can be a synonym of 'stuff')
'working at a diet's' is the wordplay.
'working' indicates an anagram.
'at'+'a'+'diets'='atadiets'
'atadiets' is an anagram of 'SATIATED'.
'getting' is the link.
(Other definitions for satiated that I've seen before include "Fully satisfied" , "Replete or surfeited" , "Stuffed with food, completely full" , "Too full, having eaten excessively" , "Filled to satisfaction" .)