Seat is for flat, used regularly (4)
I believe the answer is:
sofa
'seat' is the definition.
(I know that Sofa is a type of seat)
'is for flat used regularly' is the wordplay.
'used regularly' means one should take alternating letters.
'is'+'for'+'flat'='isforflat'
The alternate letters of 'isforflat' are 'SOFA'.
(Other definitions for sofa that I've seen before include "Upholstered seat for more than one person" , "Wide padded seat" , "Oafs (anag.)" , "Lounge" , "Long upholstered seat with back and arms" .)