Sick following doctor's practice (5)
I believe the answer is:
drill
'practice' is the definition.
('drill' can be a synonym of 'practice')
'sick following doctor's' is the wordplay.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'following' is a charade indicator (letters next to each other).
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'ill' put after 'dr' is 'DRILL'.
(Other definitions for drill that I've seen before include "Mollusc that preys on oysters" , "it could get boring" , "primate is > bore" , "Tool for boring holes" , "boring one" .)