One can see it's just a game (4)

'just a game' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'one can see' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'see' could be 'spy' (spy can mean to see or observe) and 'spy' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

'it's' acts as a link.

Can you help me to learn more?