Jack, a lover of lean little fish (5)
I believe the answer is:
sprat
'little fish' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for sprat that I've seen before include "Small sea fish" , "Small marine fish" , "Couple sharing meat" , "Small herring, often smoked or canned" , "one on a fat-free diet" .)