Ridge on a guitar's fingerboard (4)
I believe the answer is:
fret
'ridge on a guitar's fingerboard' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fret that I've seen before include "Chafe, be anxious" , "Be constantly anxious" , "Rub - a bit of the guitar" , "Show anxiety" , "Become unhappy, worry" .)