Jack who could eat no fat (5)
I believe the answer is:
sprat
'jack who could eat no fat' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sprat that I've seen before include "Small herring, often smoked or canned" , "It's set to catch a mackerel" , "Jack, perhaps" , "Traps for small fish" , "Small marine fish" .)