Study of gardening? (3)
I believe the answer is:
den
'study' is the definition.
(den can mean an office or study)
'of gardening?' is the wordplay.
'of' says the answer is hidden in the clue.
'DEN' is hidden in the letters of 'gardening'.
(Other definitions for den that I've seen before include "Wild animal's home" , "Animal's lair or gent's study" , "Private office" , "Animal's lair or man's study" , "Cave" .)