Covering Scottish headwear up (3)
I believe the answer is:
mat
'covering' is the definition.
(I know that mat is a type of covering)
'scottish headwear up' is the wordplay.
'scottish headwear' becomes 'tam' (I've seen this in another clue).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'tam' reversed gives 'MAT'.
(Other definitions for mat that I've seen before include "Small protective floor-covering" , "Small carpet" , "Thick flat pad on the floor" , "Dull, rug" , "Tangle" .)