Finally giving instrument back for nothing (6)
I believe the answer is:
gratis
'nothing' is the definition.
(I've seen this before)
'finally giving instrument back' is the wordplay.
'finally' indicates one should take the final letters.
'instrument' becomes 'sitar' (musical instrument).
'back' is a reversal indicator.
The final letter of 'giving' is 'g'.
'sitar' backwards is 'ratis'.
'g'+'ratis'='GRATIS'
'for' acts as a link.
(Other definitions for gratis that I've seen before include "Without charge" , "Free (Latin)" , "not charged" , "Free, costs nothing" , "Free, at no cost" .)