Doctor is sick from practice (5)
I believe the answer is:
drill
'practice' is the definition.
(I know that practice can be written as drill)
'doctor is sick' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'is sick' becomes 'ill' (synonyms).
'dr'+'ill'='DRILL'
'from' is the link.
(Other definitions for drill that I've seen before include "Procedure - practice" , "Instruct" , "Dental assistant" , "Square-bashing" , "Rehearsal of the procedure to be followed in an emergency" .)