Single person pretty much against marriage (5)
I believe the answer is:
union
'marriage' is the definition.
(I know that marriage can be written as union)
'single person pretty much against' is the wordplay.
'single person' becomes 'unit' (I've seen this in another clue).
'pretty much' means to remove the last letter.
'against' becomes 'on' (on can mean by or up against).
'unit' with its final letter removed is 'uni'.
'uni'+'on'='UNION'
(Other definitions for union that I've seen before include "Togetherness" , "Workers' organisation or marriage" , "Wedding, say" , "Agreement -- wedlock" , "Marriage or alliance" .)