Bad luck? Slap this on your head (8)
I believe the answer is:
skullcap
'this on your head' is the definition.
(a skullcap is worn on one's head)
'bad luck? slap' is the wordplay.
'bad' indicates anagramming the letters.
'luck'+'slap'='luckslap'
'luckslap' anagrammed gives 'SKULLCAP'.
(Other definitions for skullcap that I've seen before include "protective helmet" , "Plant" , "Close-fitting headgear with no peak for Jewish men" , "Round brimless hat worn on the crown of the head" , "Yarmulka - helmet-shaped flower" .)