Pressure from unionist wearing frock (6)
I believe the answer is:
duress
'pressure' is the definition.
(thesaurus)
'unionist wearing frock' is the wordplay.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'wearing' is an insertion indicator.
'frock' becomes 'dress' ('dress' can be a synonym of 'frock').
'u' going into 'dress' is 'DURESS'.
'from' acts as a link.
(Other definitions for duress that I've seen before include "Threat of coercion" , "Excessive pressure" , "Constraint, force" , "Threats, violence, etc, used in coercion" , "Compulsion by use of force" .)