Hat edge (4)
I believe the answer is:
brim
'hat edge' 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 "Peak (of cap); be very full" , "Edge around a hat" , "Lip; part of 6 Down" , "Cup's lip" , "Top edge of cup etc" .)