The most common and most likely answer to this crossword clue is the 7 letter word LUTHIER
I've seen this crossword clue in The Times
L
U
T
H
I
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R
More clues leading to the result LUTHIER
Buddy explains!

The straight part
We know that "who will fret?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case who will fret? = luthier.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 LUTHIER mean?
LUTHIER noun- A person who, or a business which, makes or repairs stringed wooden musical instruments, such as lutes, violins, and guitars.