Oscar crashing doctor's Beetle (3)
I believe the answer is:
dor
'beetle' is the definition.
(I've seen this in another clue)
'oscar crashing doctor's' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'crashing' indicates putting letters inside (inserted letters crash into another word).
'doctor' becomes 'dr' (abbreviation).
'o' inserted into 'dr' is 'DOR'.
(Other definitions for dor that I've seen before include "Creature" , "Dung-beetle" , "European dung - beetle" .)