Some of the upper crust is backing a Liberal (4)
I believe the answer is:
sial
'some of the upper crust' is the definition.
(material forming part of the Earth's crust)
'is backing a liberal' is the wordplay.
'backing' shows that the letters should be reversed in order.
'a liberal' becomes 'al' (I've seen this in other clues).
'is' back-to-front is 'si'.
'si'+'al'='SIAL'
(Other definitions for sial that I've seen before include "bit of crust" , "Light crust" .)