The ultimate in diet -- bread and fish (5)
I believe the answer is:
troll
'fish' is the definition.
(thesaurus)
'the ultimate in diet bread' is the wordplay.
'the ultimate in' says to take the final letters.
'bread' becomes 'roll' (I've seen this before).
The final letter of 'diet' is 't'.
't'+'roll'='TROLL'
'and' acts as a link.
(Other definitions for troll that I've seen before include "Online pest" , "Ugly Norse dwarf" , "supposedly lives in a cave" , "In Scandinavia, a mythical giant or dwarf" , "Folklore monster" .)