Game in trap with strange marks and unknown (3,5)
I believe the answer is:
gin rummy
'game' is the definition.
(card game)
'trap with strange marks and unknown' is the wordplay.
'trap with' becomes 'gin' (gin trap is a kind of animal trap. I am not sure about the 'with' bit.).
'strange' becomes 'rum' (synonyms).
'marks' becomes 'M' (former German currency).
'and' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'gin'+'rum'+'m'+'y'='GIN RUMMY'
'in' is the link.
(Other definitions for gin rummy that I've seen before include "cards" , "Card game" .)