Self-assured, able to absorb study after study (9)
I believe the answer is:
confident
'self-assured' is the definition.
(similar in meaning)
'able to absorb study after study' is the wordplay.
'able' becomes 'fit' (similar in meaning).
'to absorb' indicates putting letters inside.
'study' becomes 'den' (term for a study or office).
'after' means one lot of letters go next to another.
'study' becomes 'con' (con can informally mean to study).
'fit' enclosing 'den' is 'fident'.
'fident' after 'con' is 'CONFIDENT'.
(Other definitions for confident that I've seen before include "Feeling certain" , "Assured, positive" , "5 [CERTAIN]" , "Bold" , "Certain, sure" .)