The most common and most likely answer to this crossword clue is the 8 letter word RITENUTO
I've seen this crossword clue in Independent
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More clues leading to the result RITENUTO
- Slowing down of religious service by one who's eccentric and old
- Possible order for trio to hold melody back
- Ceremony — head getting old is slowing down
- Instruction to musicians playing trio to hold melody back?
- Musical direction we hear written out badly
- Slowing down, head appearing after ceremony’s over
- Ceremonial followed by head, old and slowing down
- Restrained ceremony offered by Brazil, say, with love
- Ceremony with character adds to a sudden slowdown
Buddy explains!

The straight part
We know that "Instruction to musicians" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instruction to musicians = ritenuto.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 RITENUTO mean?
RITENUTO noun- A musical passage of this kind.
- (of a passage in a musical score) immediately slower, held back at a slower rate of pace.