Polish face of yellow gemstone (4)
I believe the answer is:
ruby
'gemstone' is the definition.
(ruby is a kind of gemstone)
'polish face of yellow' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny**).
'face of' says to take the initial letters.
The initial letter of 'yellow' is 'y'.
'rub'+'y'='RUBY'
(Other definitions for ruby that I've seen before include "Fortieth anniversary stone" , "Red precious stone" , "Completely" , "Deep red variety of corundum" , "Jack perhaps" .)