Delight in prescribing leeches (4)
I believe the answer is:
glee
'delight' is the definition.
(glee can mean great joy)
'in prescribing leeches' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GLEE' can be found hidden inside 'prescribing leeches'.
(Other definitions for glee that I've seen before include "Great delight; part song" , "Sort of club" , "Mirth - part-song" , "Part-song for male voices - delight" , "Lively or triumphant joy" .)