Arrange mat onto seat (7)
I believe the answer is:
ottoman
'seat' is the definition.
(ottoman is a kind of seat)
'arrange mat onto' is the wordplay.
'arrange' is an anagram indicator.
'mat'+'onto'='matonto'
'matonto' is an anagram of 'OTTOMAN'.
(Other definitions for ottoman that I've seen before include "Turkish (hist.)" , "Turkish empire or ling cushioned seat" , "Chest padded as seat" , "Type of seat; Turkish" , "that lacks arms and backing" .)