Bound to affect diet (4)
I believe the answer is:
tied
'bound' is the definition.
('tie' can be a synonym of 'bind')
'affect diet' is the wordplay.
'affect' is an anagram indicator (I've seen 'affecting' mean this).
'diet' with letters rearranged gives 'TIED'.
'to' acts as a link.
(Other definitions for tied that I've seen before include "limited" , "Fastened with string" , "Drew; bound" , "Roped into diet" , "(Of pub) controlled by a brewery" .)