With "It's a deal!" getting on again inside (6)

I believe the answer is:
agreed
'with it's a deal' is the definition.
I can't judge whether this defines the answer.
'getting on again inside' is the wordplay.
'getting on' becomes 'aged' (getting on can mean getting older).
'again' becomes 're' (prefix meaning 'again').
'inside' indicates putting letters inside.
'aged' going around 're' is 'AGREED'.
Can you help me to learn more?
(Other definitions for agreed that I've seen before include "Concurred - tallied" , "Accorded" , "Expressed willingness" , "matched" , "Of one mind" .)
