Edge of a hat (4)
I believe the answer is:
brim
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">edge of a hat</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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)'" .)