Gemstone’s old, needing polish put back (4)
I believe the answer is:
opal
'gemstone's' is the definition.
(opal is a kind of gemstone)
'old needing polish put back' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'needing' says to put letters next to each other.
'polish' becomes 'lap' (to lap can mean to polish a material).
'put back' shows that the letters should be reversed in order.
'lap' written backwards gives 'pal'.
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "Glassy gemstone" , "raw material" , "Form of hydrated silica, often used as a gemstone" , "Mineral, usually iridescent" , "Gem with delicate colours" .)