Can't think what to say when Irish party holds line (3,2)
I believe the answer is:
dry up
'can't think what to say' is the definition.
'irish party holds line' is the wordplay.
'irish party' becomes 'dup' (Democratic Unionist Party).
'holds' means one lot of letters goes inside another.
'line' becomes 'ry' (abbreviation for railway).
'dup' going around 'ry' is 'DRY UP'.
'when' is the link.
(Other definitions for dry up that I've seen before include "(Of flow) decrease and stop" , "lend a hand in the scullery" , "Stop talking; fail (as a spring)" , "Use a tea towel?" , "Rude command to stop talking, or polish dishes after washing" .)