The most common and most likely answer to this crossword clue is the 6 letter word SONNET
I've seen this crossword clue in The Guardian
S
O
N
N
E
T
More clues leading to the result SONNET
- Short ditty, catch or verse
- Poem produced by boy on web
- Boy with fishing gear, eg lines set on Westminster Bridge
- Lines up outside No 10 — speech just beginning
- Issue 10 is about poetry
- A number of numbers written up, three quatrains and a couplet?
- 1 Lines from number, virtually completed, something catchy?
- Poem shown in text at regular intervals
- Issue clear in the Bard’s work
- Short lyric, final work of poet
Buddy explains!

The straight part
We know that "Lines" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lines = sonnet.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 SONNET mean?
SONNET noun- A fixed verse form of Italian origin consisting of fourteen lines that are typically five-foot iambics and rhyme according to one of a few prescribed schemes.
- To compose sonnets.
- To celebrate in sonnets; to write a sonnet about.