Worry about ridge on mandolin, for instance (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'ridge on mandolin for instance' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found 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 "Fog" , "Worry or agitate" , "Be anxious; ornament" , "Display anxiety" , "Worry - decoration - part of guitar" .)