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' says the letters should be written in reverse (letters revolve around).
'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" .)