One hand is flat (8)
I believe the answer is:
straight
This clue is a double definition.
'one hand' is the first definition.
(poker)
'flat' is the second definition.
(synonyms)
'is' is the link.
(Other definitions for straight that I've seen before include "Respectable" , "Not curved or bent" , "Good poker hand" , "Uncurved" , "proper place" .)