Restricted by a change of diet (4)
I believe the answer is:
tied
'restricted' is the definition.
(tying is a kind of restricting)
'a change of diet' is the wordplay.
'a change of' indicates an anagram (the letter order is changed).
'diet' is an anagram of 'TIED'.
'by' is the link.
(Other definitions for tied that I've seen before include "have equal scores" , "(Of pub) owned and run by a brewery" , "Made bow" , "Made into a knot" , "Fastened with string" .)