Polish stone endlessly (3)
I believe the answer is:
rub
'polish' is the definition.
(polish meaning rub to make shiny)
'stone endlessly' is the wordplay.
'stone' becomes 'ruby' (I've seen this before).
'endlessly' means to remove the last letter.
'ruby' with its final letter taken off is 'RUB'.
(Other definitions for rub that I've seen before include "Ointment; copy (a brass)" , "Irritate-...the wrong way" , "There's the . . ., a difficulty or impediment" , "Copy (a church brass)" , "Polish by friction" .)