More completely occupied (6)

I believe the answer is:

fuller

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

'more' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'completely occupied' is the wordplay.
'completely' becomes 'full' (full could historically mean fully).
'occupied' becomes 'er' (I can't justify this - if you can you should believe this answer much more).
'full'+'er'='FULLER'

Can you help me to learn more?

(Another definition for fuller that I've seen is " Less empty".)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app