Fine soldiers knocking on walls (6)
I believe the answer is:
agreed
'fine' is the definition.
(both can signal agreement when spoken)
'soldiers knocking on walls' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'knocking on' becomes 'aged' (becoming old).
'walls' indicates putting letters inside (to 'wall' can mean to contain).
're' going inside 'aged' is 'AGREED'.
(Other definitions for agreed that I've seen before include "In harmony" , "Okayed" , "Corresponded" , "OK, fine!" , "Held the same opinion" .)