Retro game incorporating third of cards in deck (5)
I believe the answer is:
orlop
'deck' is the definition.
(a deck on a ship)
'retro game incorporating third of cards' is the wordplay.
'retro' says the letters should be written backwards (retro is Latin for 'backwards').
'game' becomes 'polo' (polo is a kind of game).
'incorporating' indicates putting letters inside.
'third of cards' becomes 'r' (lrd letter of 'cards').
'polo' enclosing 'r' is 'polro'.
'polro' reversed gives 'ORLOP'.
'in' acts as a link.
(Other definitions for orlop that I've seen before include "deck at the lowest point" , "Lowest deck of a wooden sailing ship" , "Ship's lowest deck" , "hold covering" , "part of ship" .)