Dismiss objection to old instrument (7)
I believe the answer is:
sackbut
'old instrument' is the definition.
(I've seen this before)
'dismiss objection' is the wordplay.
'dismiss' becomes 'sack' (synonyms).
'objection' becomes 'but' (as in 'no ifs, no buts').
'sack'+'but'='SACKBUT'
'to' acts as a link.
(Other definitions for sackbut that I've seen before include "antique instrument" , "music-maker" , "This was in the wind" , "Old trombone" , "one on the slide" .)