For no brass instrument, note written up (6)
I believe the answer is:
gratis
'for no brass' is the definition.
(gratis means for no money or 'brass')
'instrument note written up' is the wordplay.
'instrument' becomes 'sitar' (sitar is a kind of instrument).
'note' becomes 'g' (musical note).
'written up' is a reversal indicator.
'sitar'+'g'='sitarg'
'sitarg' written backwards gives 'GRATIS'.
(Other definitions for gratis that I've seen before include "Free, costs nothing" , "Free of charge" , "Costing nothing, free" , "Without payment" , "On the house" .)