Lines adopted by party harden (3,2)
I believe the answer is:
dry up
'harden' is the definition.
(harden can mean to set or dry up)
'lines adopted by party' is the wordplay.
'lines' becomes 'ry' (abbreviation for railway).
'adopted by' is an insertion indicator (I've seen this in other clues).
'party' becomes 'dup' (Democratic Unionist Party).
'ry' put within 'dup' is 'DRY UP'.
(Other definitions for dry up that I've seen before include "Lose moisture? -shut your mouth!" , "Rude command to stop talking, or polish dishes after washing" , "work in kitchen" , "Become waterless, or rude command to stop talking" , "Use a tea towel?" .)