Bone up on what could be James Dean's headwear? (7)
I believe the answer is:
biretta
'headwear?' is the definition.
(I have seen 'religious headwear' mean 'biretta' so perhaps 'headwear' could also mean 'biretta')
'bone up on what could be' is the wordplay.
'bone' becomes 'rib' (rib is a type of bone).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'what could be' becomes 'etta' (I am not sure about this - if you are sure you should believe this answer much more).
'rib' backwards is 'bir'.
'bir'+'etta'='BIRETTA'
'james dean's' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for biretta that I've seen before include "Stiff cap worn by Roman Catholic clergy" , "ecclesiastical headgear" , "Clergyman's cap" , "Cleric takes this on himself" , "that covers a Catholic priest" .)