One object of alliance (5)
I believe the answer is:
'alliance' is the definition.
'one object' is the wordplay.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'object' becomes 'y' (Y is an example**).
'of' is the link.
(Other definitions for unity that I've seen before include "Accord" , "Completeness (with nothing wanting)" , "State of being joined as a whole" , "Mitford girl" , "A single whole" .)