Chief of staff blasted lenient guard (8)
I believe the answer is:
sentinel
'guard' is the definition.
(sentinel is a kind of guard)
'chief of staff blasted lenient' is the wordplay.
'chief of' suggests taking the first letters.
'blasted' indicates an anagram.
The first letter of 'staff' is 's'.
'lenient' anagrammed gives 'entinel'.
's'+'entinel'='SENTINEL'
(Other definitions for sentinel that I've seen before include "Look-out, military guard" , "Guardsman" , "Guard - ten lines (anag)" , "Sentry" , "One watches" .)