Old dressing on, certainly! (6)
I believe the answer is:
'certainly' is the definition.
(both can be used to show agreement)
'old dressing on' is the wordplay.
'old' becomes 'aged' (similar in meaning).
'dressing' is an insertion indicator (inserted letters are dressed or clad in others).
'on' becomes 're' ('on' can be mean 'with reference to').
'aged' enclosing 're' is 'AGREED'.
(Other definitions for agreed that I've seen before include "Harmonised" , "Jointly decided" , "granted" , "Held the same opinion" , "Settled by common consent" .)