Seen when chins drop? (4,5)

I believe the answer is:

long faces

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

'chins drop?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'seen when' is the wordplay.
I cannot really understand how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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