Soft flat hat (5)
I believe the answer is:
beret
'soft flat hat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for beret that I've seen before include "Felt hat" , "Flattish round hat" , "Flattish felt cap" , "Soft brimless cap" , "Round, brimless cap" .)