Worry about line on neck (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'about line on neck' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'line' means one lot of letters goes inside another.
'on neck' becomes 'ft' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
're' going within 'ft' is 'FRET'.
Can you help me to learn more?
(Other definitions for fret that I've seen before include "Split up" , "Worry or agitate" , "Wear; guitar ridge" , "Worry excessively" , "Worry - ridge on guitar's fingerboard" .)