Worry about the guitar part? (4)
I believe the answer is:
fret
I believe this clue is a double definition.
'worry' is the first definition.
(to fret about something is to worry)
'the guitar part?' is the second definition.
(fret is a part of a guitar)
'about' acts as a link.
(Other definitions for fret that I've seen before include "Chafe; worry" , "Worry - pine" , "Worry about doing woodwork?" , "Anguish" , "Become unhappy, worry" .)