A morning or afternoon jam session? (4,4)

I believe the answer is:

rush hour

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'jam session?' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for rush hour that I've seen before include "having battled through this?" , "Time of day when traffic is busiest" , "Busy time on roads" , "'Time of most road traffic (4,4)'" , "Traffic-snarl period" .)

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

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