Doctor with letter to the queen's surgeon's assistant (7)
I believe the answer is:
dresser
'surgeon's assistant' is the definition.
(I've seen this before)
'doctor with letter to the queen's' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'with' means one lot of letters go next to another.
'letter' becomes 'ess' (the letter S).
'to the queen' becomes 'ER' (Elizabeth Regina ie Queen Elizabeth).
'dr'+'ess'+'er'='DRESSER'
(Other definitions for dresser that I've seen before include "Theatre worker back-stage" , "One who treats wounds" , "clotheshorse?" , "Person working in one theatre" , "Piece of kitchen furniture" .)