A group of soldiers in welcome treaty (9)
I believe the answer is:
agreement
'treaty' is the definition.
(I've seen this before)
'a group of soldiers in welcome' is the wordplay.
'group of soldiers' becomes 'men' (men can mean soldiers).
'in' means one lot of letters goes inside another.
'welcome' becomes 'greet' (I've seen this in another clue).
'men' put into 'greet' is 'greement'.
'a'+'greement'='AGREEMENT'
(Other definitions for agreement that I've seen before include "in sympathy" , "match" , "State of being consistent" , "Pact" , "Harmony of opinion" .)