Oscar crashing doctor's Beetle (3)
I believe the answer is:
dor
'beetle' is the definition.
(I've seen this before)
'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' going inside 'dr' is 'DOR'.
(Other definitions for dor that I've seen before include "Creature" , "Dung-beetle" , "European dung - beetle" .)