A little exercise to keep out of bed for (3-2)

I believe the answer is:

sit-up

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'a little exercise' is the definition.
'sit up' can be an answer for 'exercise' (I've seen this before). I am not sure about the 'a little' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for sit-up that I've seen before include "Exercise designed to strengthen stomach muscles" , "Physical exercise" , "Exercise done on the floor" .)

I've seen this clue in the Evening Standard.

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