70's surfing film that's famous today (3,9)

'70's surfing film' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'famous today' is the wordplay.
I cannot really see how this works, but
'famous' could be 'big' (I've seen this in another clue) and 'big' is found within the answer.
The remaining letters 'wednesday' 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).

'that's' is the link.

Can you help me to learn more?