Protect and toughen around top of skull (6)
I believe the answer is:
insure
'protect' is the definition.
(I know that insure is a kind of protect)
'toughen around top of skull' is the wordplay.
'toughen' becomes 'inure' (I've seen this in another clue).
'around' is an insertion indicator.
'top of' says to take the initial letters.
The initial letter of 'skull' is 's'.
'inure' going around 's' is 'INSURE'.
'and' acts as a link.
(Other definitions for insure that I've seen before include "Guarantee; make secure" , "Rue sin (anag)" , "Arrange for payment in the event of loss" , "Protect by taking out a policy" , "Protect value through contract" .)