Club — United, say — regressing (5)
I believe the answer is:
wedge
'club' is the definition.
(type of golf club)
'united say regressing' is the wordplay.
'united' becomes 'we'd' (united in marriage).
'say' becomes 'eg' (both can mean 'for example').
'regressing' is a reversal indicator (to regress is to go backwards).
'eg' backwards is 'ge'.
'wed'+'ge'='WEDGE'
(Other definitions for wedge that I've seen before include "Tapered block" , "Tapered insert" , "Chock" , "Lofting golf club" , "Slice" .)