The most common and most likely answer to this crossword clue is the 8 letter word WARHORSE
I've seen this crossword clue in The Daily Telegraph
W
A
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More clues leading to the result WARHORSE
- Old, much performed piece? Actually, a modern hit
- Veteran soldier’s not so good without a right hand
- Nag in a fight
- A queen needing husband in inferior play
- Veteran soldier was hero, shot crossing river
- Arab in conflict?
- Novelist sounding harsh, we hear, in familiar piece
- In variety show, rare song often performed
- Who rears mongrel? Vet
- Who rears unexpectedly? I’d better not, in battle
Buddy explains!

The straight part
We know that "song often performed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case song often performed = warhorse.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 WARHORSE mean?
WARHORSE noun- Any horse used in horse-cavalry, but especially one bearing an armored knight.
- A regularly revived theatrical or musical work, as with Hamlet or a Beethoven symphony, or as excerpts thereto. May imply that the work in question has become hackneyed.
- An experienced person who has been through many battles, situations or contests; someone who has given long service.