Crossword Genius

Extremely short of space (4,2)

Ross

I believe the answer is:

full up

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

'short of space' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'extremely' is the wordplay.
I cannot quite understand how this works, but
'extremely' could be 'full' ('full' can be a synonym of 'extremely') and 'full' is found in the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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