Nearly everyone in union has parity (8)
I believe the answer is:
equality
'parity' is the definition.
(I've seen this before)
'nearly everyone in union' is the wordplay.
'nearly' means to remove the last letter.
'everyone' becomes 'all' (all people).
'in' means one lot of letters goes inside another.
'union' becomes 'equity' (I've seen this before).
'all' with its last letter taken away is 'al'.
'al' inserted within 'equity' is 'EQUALITY'.
'has' is the link.
(Other definitions for equality that I've seen before include "treating the duke and the dustman alike?" , "State of being the same in quantity, value or status" , "Equivalence, par" , "correspondence" , "Equivalence,par" .)