Group of soldiers in army secures agreements (8)
I believe the answer is:
treaties
'agreements' is the definition.
(I know this)
'group of soldiers in army secures' is the wordplay.
'group of soldiers' becomes 'RE' (abbreviation for Royal Engineers).
'in' means one lot of letters goes inside another.
'army' becomes 'TA' (abbreviation for Territorial Army).
'secures' becomes 'ties' (tying is a kind of securing).
're' put within 'ta' is 'trea'.
'trea'+'ties'='TREATIES'
(Other definitions for treaties that I've seen before include "Written agreements between states" , "Accords" , "Treatise about pacts" , "Rome and Versailles, for example" , "Treatise on international agreements" .)