Invaders playing cards after six (7)
I believe the answer is:
vikings
'invaders' is the definition.
(I've seen this before)
'playing cards after six' is the wordplay.
'playing cards' becomes 'kings' (king is a kind of playing card).
'after' says to put letters next to each other.
'six' becomes 'VI' (Roman numerals).
'kings' after 'vi' is 'VIKINGS'.
(Other definitions for vikings that I've seen before include "Invaders" , "Norsemen" , "Scandinavian marauders" , "Scandinavian adventurers who attacked Ireland" .)