Gem given polish, sparkly finally (4)
I believe the answer is:
ruby
'gem' is the definition.
(ruby is a kind of gem)
'polish sparkly finally' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny**).
'finally' suggests the final letters.
The final letter of 'sparkly' is 'y'.
'rub'+'y'='RUBY'
'given' is the link.
(Other definitions for ruby that I've seen before include "Stony girl" , "Deep 6 across" , "A red jewel" , "Completely" , "Girl's name - colour" .)