I'll get into doctors' exercises (6)
I believe the answer is:
drills
'exercises' is the definition.
('drill' can be a synonym of 'exercise')
'i'll get into doctors'' is the wordplay.
'get into' indicates putting letters inside.
'doctors'' becomes 'Drs' ('Dr' can be a synonym of 'doctor').
'ill' inserted within 'drs' is 'DRILLS'.
(Other definitions for drills that I've seen before include "monkeys" , "Boring tools" , "Trains, in military fashion" , "Teaches by repetition" , "Tools with a sharp points for making holes" .)