Guitar part causing worry (4)
I believe the answer is:
fret
I believe this clue is a double definition.
'guitar part' is the first definition.
(fret is a part of a guitar)
'worry' is the second definition.
(to fret about something is to worry)
'causing' acts as a link.
(Other definitions for fret that I've seen before include "Split up" , "coastal mist" , "Worry - bar on the fingerboard of a stringed instrument" , "Worry - ridge on guitar's fingerboard" , "Harry - on the guitar" .)