Learn to cover bum, having first removed hat? (8)
I believe the answer is:
headgear
'hat?' is the definition.
(hat is a kind of headgear)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for headgear that I've seen before include "Aged hare? (anagram)" , "bowler, for example" , "helmet, for instance" , "Mortarboard < , especially" , "Hats, helmets etc." .)