Polish round back for gemstone (4)
I believe the answer is:
opal
'gemstone' is the definition.
(opal is a kind of gemstone)
'polish round back' is the wordplay.
'polish' becomes 'lap' (to lap can mean to polish a material).
'round' becomes 'o' (round can mean a round object).
'back' is a reversal indicator.
'lap'+'o'='lapo'
'lapo' reversed gives 'OPAL'.
'for' is the link.
(Other definitions for opal that I've seen before include "raw material" , "Milky stuff that's hard" , "Semi-precious stone of milky or bluish colour" , "Something milky-white" , "Iridescent quartz-like stone" .)