Companion who may be sleeping? (7)

I believe the answer is:

partner

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

'companion' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for partner that I've seen before include "Other half?" , "Sidekick" , "sometimes joining in crime" , "Either member of a couple" , "south to north, perhaps" .)

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