Quite realising you can't squeeze another thing in? (5,5)

I believe the answer is:

fully aware

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

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

'you can't squeeze another thing in?' is the wordplay.
I cannot really see how this works, but
'you' could be 'u' (commonly used in online communication) and 'u' is found in the answer.
'another' could be 're' (I've seen this in other clues) and 're' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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