Headwear that's uncovered (3)
I believe the answer is:
hat
'headwear' is the definition.
(hat is a type of headwear)
'that's uncovered' is the wordplay.
'uncovered' indicates a hidden word.
'HAT' is hidden within 'thats'.
(Other definitions for hat that I've seen before include "One to cover head" , "Headwear" , "Headdress" , "Bowler maybe" , "Eg, derby" .)