Edge of a hat (4)
I believe the answer is:
brim
'edge of a hat' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for brim that I've seen before include "Lip; part of 6 Down" , "Lip of a vessel" , "Top edge of a container" , "Top edge of cup etc" , "'Edge, like that of a hat (4)'" .)