Irritating incident that can harm you at first (5)
I believe the answer is:
itchy
'irritating' is the definition.
(an itch is irritating)
'incident that can harm you at first' is the wordplay.
'at first' says to take the initial letters.
The first letters of 'incident that can harm you' is 'ITCHY'.
(Other definitions for itchy that I've seen before include "Unable to relax" , "Making one want to scratch" , "Tickling" , "Requiring a scratch" , "Irritable" .)