Popular legislation is relative where union is concerned (2-3)
I believe the answer is:
in-law
'relative where union is concerned' is the definition.
(union can refer to a marriage)
'popular legislation' is the wordplay.
'popular' becomes 'in'.
'legislation' becomes 'law' (I've seen this before).
'in'+'law'='IN-LAW'
'is' acts as a link.
(Other definitions for in-law that I've seen before include "Meghan to Harry's family?" , "Near relative" , "Spouse's relation" , "Relation by marriage" , "Family member, by marriage" .)