One could be on the way out (4,4)

'one' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'be on the way out' is the wordplay.
'be' becomes 'sign' (I can't justify this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another.
'the way out' becomes 'exit' ('exit' can be a synonym of 'way out').
'sign' after 'exit' is 'EXIT SIGN'.

'could' is the link.

Can you help me to learn more?