Outfit absorbing unionist pressure (6)
I believe the answer is:
duress
'pressure' is the definition.
(thesaurus)
'outfit absorbing unionist' is the wordplay.
'outfit' becomes 'dress' (I've seen this in other clues).
'absorbing' means one lot of letters goes inside another.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'dress' enclosing 'u' is 'DURESS'.
(Other definitions for duress that I've seen before include "Hardship" , "The use of force or threats to get one's way" , "Force, coercion" , "Use of force or threats to get something" , "Use of force or threats, often vile" .)