Graduate before nightfall? Goes with swimmers? (7,5)

I believe the answer is:

bathing suits

I'm a little stuck... Click here to teach me more about this clue!

'swimmers?' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.

'graduate before nightfall? goes' is the wordplay.
I cannot really understand how this works, but
'graduate' could be 'ba' (someone with a Bachelor of Arts degree) and 'ba' is found within the answer.
This explanation may well be incorrect...

'with' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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