Grave, sort of square old doctor (4)
I believe the answer is:
tomb
'grave' is the definition.
('tomb' can be a synonym of 'grave')
'sort of square old doctor' is the wordplay.
'sort of square' becomes 't' (a T-Square is a technical drawing instrument).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
't'+'o'+'mb'='TOMB'
(Other definitions for tomb that I've seen before include "Mausoleum" , "Place of burial, grave or vault" , "Burial site" , "Building for a body" , "Burial-vault" .)