The most common and most likely answer to this crossword clue is the 7 letter word RHYMING
I've seen this crossword clue in The Daily Telegraph
R
H
Y
M
I
N
G
Buddy explains!
The straight part
We know that "element of poetry and slang?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case element of poetry and slang? = rhyming.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 RHYMING mean?
RHYMING verb- To compose or treat in verse; versify.
- (followed by with) Of a word, to be pronounced identically with another from the vowel in its stressed syllable to the end.
- Of two or more words, to be pronounced identically from the vowel in the stressed syllable of each to the end of each.
- To number; count; reckon.
- Rhyme