Revolutionary Indian instrument good for nothing (6)
I believe the answer is:
gratis
'nothing' is the definition.
(I've seen this before)
'revolutionary indian instrument good' is the wordplay.
'revolutionary' is a reversal indicator.
'indian instrument' becomes 'sitar' (I've seen this before).
'good' becomes 'g' (abbreviation).
'sitar'+'g'='sitarg'
'sitarg' backwards is 'GRATIS'.
'for' acts as a link.
(Other definitions for gratis that I've seen before include "Without payment" , "Free, at no cost" , "Costing nothing, free" , "Free (Latin)" , "Without charge" .)