Great to study expert (12)
I believe the answer is:
considerable
'great' is the definition.
(I've seen this before)
'study expert' is the wordplay.
'study' becomes 'consider' ('consider' can be a synonym of 'study'**).
'expert' becomes 'able' (I've seen this in other clues).
'consider'+'able'='CONSIDERABLE'
'to' acts as a link.
(Other definitions for considerable that I've seen before include "worthy of attention" , "Tidy" , "worth entertaining?" , "Major" , "A lot (of) - worthy of respect" .)