Exercise makes doctor unwell (5)
I believe the answer is:
drill
'exercise' is the definition.
(I know that exercise can be written as drill)
'doctor unwell' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'unwell' becomes 'i'll' (similar in meaning).
'dr'+'ill'='DRILL'
'makes' is the link.
(Other definitions for drill that I've seen before include "Tool for boring holes" , "Tool, instruction" , "Exercise by repeated practice - bore" , "Seed machine; baboon" , "Monkey" .)