Bound to change diet (4)
I believe the answer is:
tied
'bound' is the definition.
(similar in meaning)
'change diet' is the wordplay.
'change' indicates an anagram.
'diet' with letters rearranged 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" .)