Half of Elon Musk's head is wood (3)
I believe the answer is:
elm
'is wood' is the definition.
(elm is a type of tree)
'half of elon musk's head' is the wordplay.
'half of' suggests halving the letters.
'head' suggests taking the first letters.
The initial letter of 'musks' is 'm'.
'elon' cut in half is 'el'.
'el'+'m'='ELM'
(Other definitions for elm that I've seen before include "Tree of the genus Ulmus" , "Native British tree" , "Tree for a Cockney tiller" , "Stately tree" , "Mel's tree" .)