Polish nurse once entertaining ambassador (5)
I believe the answer is:
sheen
'polish' is the definition.
(both can mean a shiny surface)
'nurse once entertaining ambassador' is the wordplay.
'nurse once' becomes 'SEN' (abbreviation for state-enrolled nurse).
'entertaining' is an insertion indicator (inviting in, as for dinner).
'ambassador' becomes 'HE' (His or Her Excellency).
'sen' placed around 'he' is 'SHEEN'.
(Other definitions for sheen that I've seen before include "Oil slick" , "actor?" , "Sureface shine" , "Superficial brightness" , "place in London" .)