The most common and most likely answer to this crossword clue is the 3 plus 2 plus 1 plus 7 plus 3 letter word ODE ON A GRECIAN URN
I've seen this crossword clue in The Times
O
D
E
O
N
A
G
R
E
C
I
A
N
U
R
N
Buddy explains!
The straight part
We know that "Lines of John’s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lines of john’s = ode on a grecian urn.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.