Training exercise is not well introduced by doctor (5)
I believe the answer is:
drill
'training exercise' is the definition.
(I've seen this before)
'not well introduced by doctor' is the wordplay.
'not well' becomes 'ill' ('ill' is the opposite to 'well').
'introduced by' is a charade indicator (letters next to each other).
'doctor' becomes 'dr' (abbreviation).
'ill' after 'dr' is 'DRILL'.
'is' acts as a link.
(Other definitions for drill that I've seen before include "Square-bashing" , "Procedure - practice" , "Hole-making tool" , "Tool for boring holes" , "Dental assistant" .)