Brood about line on neck (4)
I believe the answer is:
fret
'brood' is the definition.
(thesaurus)
'line on neck' is the wordplay.
I cannot really understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is located in the answer.
This explanation may well be incorrect...
'about' is the link.
Can you help me to learn more?
(Other definitions for fret that I've seen before include "Worry or agitate" , "Anguish" , "Vex oneself - part of a fingerboard" , "Worry about doing woodwork?" , "Show anxiety" .)