Very enthusiastic study in painting (6)
I believe the answer is:
ardent
'very enthusiastic' is the definition.
(I've seen this in another clue)
'study in painting' is the wordplay.
'study' becomes 'den' (term for a study or office).
'in' indicates putting letters inside.
'painting' becomes 'art' (painting is an art).
'den' put inside 'art' is 'ARDENT'.
(Other definitions for ardent that I've seen before include "Having great warmth of feeling like a lover" , "'Fervent, impassioned (6)'" , "passion" , "Loving and fiery" , "Burning with emotion" .)