Head of security entering by way of official pass (4)
I believe the answer is:
visa
'official pass' is the definition.
'visa' can be an answer for 'pass' (pass allowing international travel). I'm not sure about the 'official' bit.
'head of security entering by way of' is the wordplay.
'head of' indicates taking the first letters.
'entering' indicates putting letters inside.
'by way of' becomes 'via' (via means by way of or by means of).
The initial letter of 'security' is 's'.
's' put into 'via' is 'VISA'.
Can you help me to learn more?
(Other definitions for visa that I've seen before include "Travel stamp in passport" , "giving you access to capital?" , "ID" , "Travel document is required to enter Virginia" , "Travel permit stamped in passport etc" .)