Unfairness of popular union (8)
I believe the answer is:
inequity
'unfairness' is the definition.
(I know that unfairness can be written as inequity)
'popular union' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'union' becomes 'equity' (I've seen this before).
'in'+'equity'='INEQUITY'
'of' is the link.
(Other definitions for inequity that I've seen before include "Prejudice" , "not fair" , "Unjust treatment" , "Lack of fairness" , "Unfairness, injustice" .)