Guard, head of security, unusually lenient (8)
I believe the answer is:
sentinel
'guard' is the definition.
(sentinel is a kind of guard)
'head of security unusually lenient' is the wordplay.
'head of' indicates taking the first letters.
'unusually' is an anagram indicator.
The first letter of 'security' is 's'.
'lenient' anagrammed gives 'entinel'.
's'+'entinel'='SENTINEL'
(Other definitions for sentinel that I've seen before include "One stands and keeps watch" , "Watching guard" , "One watching" , "Guard whose job is to stand and keep watch" , "One on guard" .)