Athlete once trying to smuggle chocolate bars? (8,6)

'trying to smuggle chocolate bars?' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'athlete' is the wordplay.
I cannot quite see how this works, but
'athlete' could be 'runner' (type of athlete) and 'runner' is present in the answer.
The remaining letters 'marathon' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'once' acts as a link.

Can you help me to learn more?