Chaps engaged in a welcome understanding (9)
I believe the answer is:
agreement
'understanding' is the definition.
(I've seen this before)
'chaps engaged in a welcome' is the wordplay.
'chaps' becomes 'men' (I've seen this before).
'engaged in' means one lot of letters goes inside another.
'welcome' becomes 'greet' (I've seen this before).
'a'+'greet'='agreet'
'men' going into 'agreet' is 'AGREEMENT'.
(Other definitions for agreement that I've seen before include "Contract or promise" , "Unlikely result" , "consensus" , "concert" , "Harmony, concurrence" .)