Doctor to pose with chest instrument (11)
I believe the answer is:
stethoscope
'instrument' is the definition.
(stethoscope is a kind of instrument)
'doctor to pose with chest' is the wordplay.
'doctor' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'to'+'pose'='topose'
'topose' put after 'chest' is 'chesttopose'.
'chesttopose' anagrammed gives 'STETHOSCOPE'.
(Other definitions for stethoscope that I've seen before include "Medical instrument" , "Instrument used in listening" , "listening device" , "Doctor's instrument" .)