Fixed or varying diet? (4)
I believe the answer is:
tied
'fixed' is the definition.
(tying is a kind of fixing)
'varying diet?' is the wordplay.
'varying' indicates anagramming the letters.
'diet' with letters rearranged gives 'TIED'.
'or' acts as a link.
(Other definitions for tied that I've seen before include "Roped into diet" , "Restricted, bound" , "Made bow" , "Drawn" , "With an equal score" .)