Rush in with a stethoscope (5)
I believe the answer is:
haste
'rush' is the definition.
(I know that rush can be written as haste)
'in with a stethoscope' is the wordplay.
'in' indicates a hidden word.
'HASTE' is hidden within 'with a stethoscope'.
(Other definitions for haste that I've seen before include "Rush; urgency" , "Hurriedness" , "The more of it, the less speed, it is said" , "Heats (anag.)" , "Undue speed" .)