High-tech, special card allows guards entry at the front (5-3)
I believe the answer is:
space-age
'high-tech' is the definition.
'special card allows guards entry at the front' is the wordplay.
'special' becomes 'sp' (abbreviation).
'card' becomes 'ace' (ace is a type of playing card).
'at the front' indicates taking the first letters.
The first letters of 'allows guards entry' is 'age'.
'sp'+'ace'+'age'='SPACE-AGE'
(Other definitions for space-age that I've seen before include "Ultra-modern" , "Futuristic" .)