Sign of healing in Charlie’s back muscle (4)
I believe the answer is:
scab
'sign of healing' is the definition.
(I've seen this in another clue)
'charlie's back muscle' is the wordplay.
'charlie's' becomes 'c's' (phonetic alphabet: alpha, bravo, charlie etc.).
'back' says the letters should be written in reverse.
'muscle' becomes 'ab' (short for abdominal muscle).
'cs' back-to-front is 'sc'.
'sc'+'ab'='SCAB'
'in' acts as a link.
(Other definitions for scab that I've seen before include "Strikebreaker; mange" , "Dried crust on wound" , "Dried crust on sore" , "Crust on wound, or a blackleg" , "Scoundrel" .)