The most common and most likely answer to this crossword clue is the 4 plus 5 letter word JACK SPRAT
I've seen this crossword clue in The Daily Telegraph
J
A
C
K
S
P
R
A
T
Buddy explains!
The straight part
We know that "man in nursery rhyme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case man in nursery rhyme = jack 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.