Member in a register, scaremonger (8)
I believe the answer is:
alarmist
'scaremonger' is the definition.
(I've seen this before)
'member in a register' is the wordplay.
'member' becomes 'arm' (arm is a kind of member).
'in' indicates putting letters inside.
'register' becomes 'list' (listing is a kind of registering**).
'a'+'list'='alist'
'arm' going inside 'alist' is 'ALARMIST'.
(Other definitions for alarmist that I've seen before include "Person exaggerating danger" , "One overstating danger" , "prophet of doom" , "(Person) creating unnecessary panic" , "One who exaggerates a danger" .)