Mark on guitar appearing regularly in ferment (4)
I believe the answer is:
fret
'mark' is the definition.
(I know that fret is a type of dapple)
'regularly in ferment' is the wordplay.
'regularly in' indicates alternate letters.
The alternating letters of 'ferment' are 'FRET'.
'on guitar appearing' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for fret that I've seen before include "Become unhappy, worry" , "Be constantly anxious" , "Anguish" , "Harry - on the guitar" , "Worry - bar on the fingerboard of a stringed instrument" .)