Bound to change diet (4)
I believe the answer is:
'bound' is the definition.
('tie' can be a synonym of 'bind')
'change diet' is the wordplay.
'change' is an anagram indicator.
'diet' anagrammed gives 'TIED'.
'to' acts as a link.
(Other definitions for tied that I've seen before include "Drawn" , "Bound to diet" , "Roped into diet" , "Fastened, bound" , "Bound or secured" .)