Solid Tory base (4)
I believe the answer is:
cone
'solid' is the definition.
(3d shape)
'tory base' is the wordplay.
'tory' becomes 'con' (abbreviation for Conservative).
'base' becomes 'e' (Euler's number - base of natural logarithms).
'con'+'e'='CONE'
(Other definitions for cone that I've seen before include "Roadworks marker" , "Tapering solid" , "Tapering object" , "may obstruct lane" , "Solid figure" .)