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 anagramming the letters.
'diet' with letters rearranged gives 'TIED'.
'to' is the link.
(Other definitions for tied that I've seen before include "Drawn" , "Bound to diet" , "Roped into diet" , "Fastened, bound" , "Bound or secured" .)