What dentist will have done – trained? (7)
I believe the answer is:
drilled
I believe this clue is a double definition.
'what dentist will have done' is the first definition.
'trained?' is the second definition.
(drilling is a kind of training)
(Other definitions for drilled that I've seen before include "Made a hole with a pointed tool" , "given military instruction" , "Bored a hole or trained by repetition" , "soldiers may be treated thus" .)