Polish top of your gemstone (4)
I believe the answer is:
ruby
'gemstone' is the definition.
(ruby is a kind of gemstone)
'polish top of your' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny**).
'top of' indicates taking the first letters.
The initial letter of 'your' is 'y'.
'rub'+'y'='RUBY'
(Other definitions for ruby that I've seen before include "Vivid red" , "Deep red variety of corundum" , "Gemstone comprising mainly aluminium oxide" , "A red jewel" , "Completely" .)