Doctor has an attempt to take on a cavalryman (7)
I believe the answer is:
dragoon
'a cavalryman' is the definition.
(I know that dragoon is a type of cavalryman)
'doctor has an attempt to take on' is the wordplay.
'doctor has' becomes 'dra' (I can't explain this - if you can you should believe this answer much more).
'an attempt' becomes 'go' (go is a kind of attempt).
'to take' is a charade indicator (letters next to each other).
'dra'+'go'+'on'='DRAGOON'
Can you help me to learn more?
(Other definitions for dragoon that I've seen before include "Cavalry soldier" , "Press" , "Constrain" , "Coerce; mounted soldier" , "Coerce someone to do something against their will" .)