Polish base of Blarney Stone? (4)
I believe the answer is:
ruby
'stone?' is the definition.
(I've seen this in other clues)
'polish base of blarney' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'base of' says to take the final letters.
The final letter of 'blarney' is 'y'.
'rub'+'y'='RUBY'
(Other definitions for ruby that I've seen before include "Deep 6 across" , "Stony girl" , "Fortieth anniversary stone" , "Girl's name - colour" , "Gemstone - shade of red" .)