Fixed diet is poor (4)
I believe the answer is:
tied
'fixed' is the definition.
(tying is a kind of fixing)
'diet is poor' is the wordplay.
'is poor' indicates an anagram.
'diet' with letters rearranged gives 'TIED'.
(Other definitions for tied that I've seen before include "Drew; bound" , "eg Scored 1-1" , "(Of pub) controlled by a brewery" , "Bound to diet" , "Made bow" .)