Cognoscenti regularly avoided attack (5)
I believe the answer is:
onset
'attack' is the definition.
(archaic term for a military attack)
'cognoscenti regularly avoided' is the wordplay.
'regularly avoided' means one should take alternating letters (letters removed at regular intervals).
The alternate letters of 'cognoscenti' are 'ONSET'.
(Other definitions for onset that I've seen before include "Beginning (of disease etc)" , "Beginning; military attack" , "Start - notes (anag)" , "Outbreak" , "Start, beginning" .)