Old hotel tonight mistaken about number turning up (4,2,3,5)
I believe the answer is:
long in the tooth
'old' is the definition.
(I've seen this before)
'hotel tonight mistaken about number turning up' is the wordplay.
'mistaken' is an anagram indicator.
'about' means one lot of letters goes inside another.
'number' becomes 'no' (abbreviation).
'turning up' shows that the letters should be reversed in order.
'no' reversed gives 'on'.
'hotel'+'tonight'='hoteltonight'
'hoteltonight' with letters rearranged gives 'lginthetooth'.
'lginthetooth' going around 'on' is 'LONG IN THE TOOTH'.
(Other definitions for long in the tooth that I've seen before include "Rather old" , "Perhaps like a 1970s filling" , "indication of the need for cosmetic dentistry?" , "Been around" , "Was Dracula so" .)