Enquiring cry? There’s nothing in the first room (6)

'enquiring cry?' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'nothing in the first room' is the wordplay.
'nothing in' becomes 'oa' (I am not sure about this - if you are sure you should believe this answer much more).
'the first' means one lot of letters go next to another.
'room' becomes 'hall' (hall is a kind of room).
'oa' put after 'hall' is 'HALLOA'.

'there's' is the link.

Can you help me to learn more?