Pine part of guitar (4)
I believe the answer is:
fret
I believe this clue is a double definition.
'pine' is the first definition.
(fret can mean to pine away or worry)
'part of guitar' is the second definition.
(a guitar has frets)
(Other definitions for fret that I've seen before include "Wear; guitar ridge" , "Rub - a bit of the guitar" , "Fog" , "Split up" , "Worry - decoration - part of guitar" .)