Shock of member supporting a Labour leader (5)
I believe the answer is:
alarm
'shock' is the definition.
(I've seen this before)
'member supporting a labour leader' is the wordplay.
'member' becomes 'arm' (arm is a kind of member).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'leader' says to take the initial letters.
The first letter of 'labour' is 'l'.
'a'+'l'='al'
'arm' put after 'al' is 'ALARM'.
'of' is the link.
(Other definitions for alarm that I've seen before include "Agitate" , "Great concern" , "Frighten, disturb" , "Frighten, scare" , "Alternative to reveille" .)