Bound to alter diet (4)
I believe the answer is:
tied
'bound' is the definition.
('tie' can be a synonym of 'bind')
'alter diet' is the wordplay.
'alter' is an anagram indicator.
'diet' is an anagram of 'TIED'.
'to' is the link.
(Other definitions for tied that I've seen before include "Bound to diet" , "Made bow" , "With an equal score" , "Made into a knot" , "Bound or secured" .)