So the head of security gets the message (3)
I believe the answer is:
sos
'the message' is the definition.
'so the head of security' is the wordplay.
'the head of' suggests taking the first letters.
The first letter of 'security' is 's'.
'so'+'s'='SOS'
'gets' acts as a link.
(Other definitions for sos that I've seen before include "International distress code" , "Distress call" , "Coded distress signal" , "Help with dots and dashes" , "Morse Code for help" .)