Gave approval for a grand instrument (6)
I believe the answer is:
agreed
'gave approval' is the definition.
'a grand instrument' is the wordplay.
'a grand' becomes 'ag' (I've seen this in other clues).
'instrument' becomes 'reed' (reed is a kind of instrument).
'ag'+'reed'='AGREED'
'for' acts as a link.
(Other definitions for agreed that I've seen before include "Consented and concurred" , "Gave consent" , "Assented, acquiesced" , "It's a deal!" , "Jointly decided" .)