This is one party game (6)
I believe the answer is:
Here is my best explanation:
'game' is the definition.
(I've seen this before)
'this is one party' is the wordplay.
'this is one' becomes 'clue' (this clue).
'party' becomes 'do' (do is a kind of party).
(Other definitions for cluedo that I've seen before include "Detective game" and "Murder mystery board game".)