Worry line on neck (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'line on neck' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fret that I've seen before include "Rub - a bit of the guitar" , "Part of a Gibson, say" , "worry its way through?" , "Vex oneself - part of a fingerboard" , "Worry, brood" .)