Worry about line seen on neck (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'about line seen on neck' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'line' indicates putting letters inside.
'seen 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 into 'ft' is 'FRET'.
Can you help me to learn more?
(Other definitions for fret that I've seen before include "Mist coming in from the sea" , "Worry; sort of saw" , "Become unhappy, worry" , "ridge > that obscures sea view" , "Worry or agitate" .)