Sort of red polish unknown (4)
I believe the answer is:
ruby
'sort of red' is the definition.
'polish unknown' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'unknown' becomes 'y' (algebra).
'rub'+'y'='RUBY'
(Other definitions for ruby that I've seen before include "Fortieth anniversary stone" , "Gemstone - shade of red" , "Deep red variety of corundum" , "Gemstone comprising mainly aluminium oxide" , "Red precious stone" .)