The joining of two or more together (5)
I believe the answer is:
union
'the joining of two or more together' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for union that I've seen before include "The joining of two or more together" , "junction" , "A joining together as in marriage" , "Organisation of employees for bargaining purposes" , "Merger like marriage" .)