Yours truly taking forty winks at half nine on floor! (9)
I believe the answer is:
mezzanine
'floor' is the definition.
(I know that mezzanine is a type of floor)
'yours truly taking forty winks at half nine' is the wordplay.
'yours truly' becomes 'me' (the person who set this crossword).
'taking forty winks' becomes 'zz' (I can't justify this - if you can you should believe this answer much more).
'half' means to take half the letters.
'at' cut in half is 'a'.
'me'+'zz'+'a'+'nine'='MEZZANINE'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for mezzanine that I've seen before include "Floor between floors" , "Intermediate floor (usually between ground and first)" , "Intermediate storey (often between ground and first)" , "Floor immediately above ground level" , "Storey between stories" .)