Doctor possessing old Beetle (3)
I believe the answer is:
dor
'beetle' is the definition.
(I've seen this before)
'doctor possessing old' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'possessing' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'dr' enclosing 'o' is 'DOR'.
(Other definitions for dor that I've seen before include "Dung-beetle" , "Creature" , "European dung - beetle" .)