In formal dress a large number finally left, the place now empty (5,4)
I believe the answer is:
ghost town
'place now empty' is the definition.
(term for an abandoned settlement)
'in formal dress a large number finally left' is the wordplay.
'in' indicates putting letters inside.
'formal dress' becomes 'gown' (I've seen this in another clue).
'a large number' becomes 'host' (I've seen this in another clue).
'finally left' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'host'+'t'='hostt'
'gown' placed around 'hostt' is 'GHOST TOWN'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for ghost town that I've seen before include "There's nobody in it" , "where there's no-one around" , "abandoned position" , "Haunted" , "Deserted urban area" .)