Tidy flat (8)
I believe the answer is:
straight
I believe this clue is a double definition.
'tidy' is the first definition.
(thesaurus)
'flat' is the second definition.
(synonyms)
(Other definitions for straight that I've seen before include "Honest, not crooked" , "Unswerving" , "square" , "Candid" , "not gay" .)