Drill oddly turned top of skull (6)
I believe the answer is:
trepan
'drill' is the definition.
(trepan is a kind of drill)
'oddly turned top of skull' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'top of skull' becomes 'pan' (I can't justify this - if you can you should believe this answer much more).
The alternate letters of 'turned' are 'tre'.
'tre'+'pan'='TREPAN'
Can you help me to learn more?
(Other definitions for trepan that I've seen before include "Surgical saw" , "old saw" , "Make a hole in skull" , "Surgically create a hole in the skull" , "Surgical instrument used to remove sections of bone from the skull" .)