Note deserter is not charged (6)
I believe the answer is:
gratis
'not charged' is the definition.
(I've seen this before)
'note deserter is' is the wordplay.
'note' becomes 'g' (musical note).
'deserter' becomes 'rat' (both can mean a traitor).
'g'+'rat'+'is'='GRATIS'
(Other definitions for gratis that I've seen before include "Costing nothing, free" , "Free, at no cost" , "not charged" , "Without payment" , "Free, costs nothing" .)