Getting on, having provided billet for soldiers? Certainly (6)
I believe the answer is:
agreed
'certainly' is the definition.
(both can be used to show agreement)
'getting on having provided billet for soldiers?' is the wordplay.
'getting on' becomes 'aged' (getting on can mean getting older).
'having provided billet for' means one lot of letters goes inside another (some letters provide accommodation for others).
'soldiers?' becomes 'RE' (Royal Engineers).
'aged' placed around 're' is 'AGREED'.
(Other definitions for agreed that I've seen before include "matched" , "acceptable" , "Held the same opinion" , "Accorded" , "OK, fine!" .)