Sign of importance dispensing with protection (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that Omen is a type of sign)
'importance dispensing with protection' is the wordplay.
'importance' becomes 'moment' (moment is a kind of importance).
'dispensing with protection' says to take the centre (letters which protect the word are removed).
The middle of 'moment' is 'OMEN'.
'of' is the link.
(Other definitions for omen that I've seen before include "Sign of likely future event" , "Foreboding" , "Sign of something about to happen" , "Augur" , "premonition" .)