Flat topper (3)
I believe the answer is:
tam
'flat topper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tam that I've seen before include "These ratings measure TV audiences" , "Scots hat" , "Scots cap" , "Index rating for TV watching" , "'Woollen cap of Scottish origin, in short (3)'" .)