Polish gemstone endlessly (3)
I believe the answer is:
rub
'polish' is the definition.
(polish meaning rub to make shiny)
'gemstone endlessly' is the wordplay.
'gemstone' becomes 'ruby' (ruby is a kind of gemstone).
'endlessly' means to remove the last letter.
'ruby' with its last letter taken away is 'RUB'.
(Other definitions for rub that I've seen before include "Massage or buff" , "Polish - difficulty" , "Massage or burnish" , "Apply friction to - difficulty" , "There's the . . ., a difficulty or impediment" .)