Hard skull, say, white with age (4)
I believe the answer is:
hoar
'white with age' is the definition.
(I've seen this in another clue)
'hard skull' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'skull' becomes 'oar' (I can't explain this - if you can you should believe this answer much more).
'h'+'oar'='HOAR'
'say' is the link.
Can you help me to learn more?
(Other definitions for hoar that I've seen before include "Greyish-white with age or frost" , "Grey with age; frost" , "Rime" , "Type of frost" , "A kind of white frost" .)