Card game boss’s defensive position (10)
I believe the answer is:
bridgehead
'defensive position' is the definition.
(I've seen this before)
'card game boss's' is the wordplay.
'card game' becomes 'bridge' (bridge is a kind of card game).
'boss' becomes 'head' (synonyms).
'bridge'+'head'='BRIDGEHEAD'
(Other definitions for bridgehead that I've seen before include "Forward position seized by attacking troops" , "Advanced position secured in enemy territory" , "Secured position from which to advance" , "Advantageous position" , "Strong position secured inside enemy territory" .)