Unionist involved in vice? Refuse to respond (4,2)
I believe the answer is:
clam up
'refuse to respond' is the definition.
I can't judge whether this definition defines the answer.
'unionist involved in vice?' is the wordplay.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'involved in' means one lot of letters goes inside another.
'vice?' becomes 'clamp' (I've seen this in other clues).
'u' inserted within 'clamp' is 'CLAM UP'.
Can you help me to learn more?
(Other definitions for clam up that I've seen before include "Stop giving out" , "Refuse to speak" , "Abruptly stop talking (colloq.)" , "'Fall silent and refuse to talk (4,2)'" , "Go silent" .)