The most common and most likely answer to this crossword clue is the 8 letter word PALINODE
I've seen this crossword clue in The Guardian
P
A
L
I
N
O
D
E
Buddy explains!
The straight part
We know that "poem, after change of heart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case poem, after change of heart = palinode.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 PALINODE mean?
PALINODE noun- A poem in which the author retracts something said in an earlier poem.