The most common and most likely answer to this crossword clue is the 5 letter word SPRAT
I've seen this crossword clue in The Times
S
P
R
A
T
More clues leading to the result SPRAT
Buddy explains!
The straight part
We know that "Jack who’d only eat lean/fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jack who’d only eat lean/fish = sprat.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SPRAT mean?
SPRAT noun- Any of various small, herring-like, marine fish in the genus Sprattus, in the family Clupeidae.