Bound to adjust diet (4)
I believe the answer is:
tied
'bound' is the definition.
('tie' can be a synonym of 'bind')
'adjust diet' is the wordplay.
'adjust' is an anagram indicator.
'diet' with letters rearranged gives 'TIED'.
'to' acts as a link.
(Other definitions for tied that I've seen before include "Bonded" , "eg Scored 1-1" , "Restricted, bound" , "Linked; restricted" , "Knotted" .)