'Deceptively roomy' base used instead of a vacated lobby (10)

'deceptively roomy base used' is the definition.
I can't judge whether this defines the answer.

'instead of a vacated lobby' is the wordplay.
'instead of' becomes 'specious' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a vacated' means to remove the middle letters (internal letters vacate the word).
'lobby' with its centre removed is 'ly'.
'specious'+'ly'='SPECIOUSLY'

Can you help me to learn more?