Trap game under base of oak tree (6)

'tree' is the definition.
(ginkgo is a kind of tree)

'trap game under base of oak' is the wordplay.
'trap' becomes 'gin' (I've seen this before).
'game' becomes 'go' (go is a kind of game).
'under' is a charade indicator (letters next to each other).
'base of' indicates one should take the final letters.
The final letter of 'oak' is 'k'.
'go' put after 'k' is 'kgo'.
'gin'+'kgo'='GINKGO'