Flat on top? He puts one out (9)
I believe the answer is:
penthouse
'flat' is the definition.
(I know that penthouse is a type of flat)
'on top? he puts one out' is the wordplay.
'on top?' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'he'+'puts'='heputs'
'heputs'+'one' is 'heputsone'.
'heputsone' with letters rearranged gives 'PENTHOUSE'.
(Other definitions for penthouse that I've seen before include "High apartment" , "roof flat" , "Luxurious apartment on top of tall building" , "luxury property" , "Top 19 across" .)