Part of shibboleth — a little deadly (6)
I believe the answer is:
lethal
'deadly' is the definition.
('lethal' can be a synonym of 'deadly')
'part of shibboleth a little' is the wordplay.
'part of' says the answer is hidden in the clue.
'LETHAL' is hidden in the letters of 'shibboleth a little'.
(Other definitions for lethal that I've seen before include "Destructive" , "Noxious" , "Murderous" , "very nasty indeed" , "Deadly or mortal" .)