Instruction to serviceman: polish grain endlessly (6)
I believe the answer is:
rubric
'instruction to serviceman' is the definition.
'polish grain endlessly' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'grain' becomes 'rice' (rice is a kind of grain).
'endlessly' means to remove the last letter.
'rice' with its last letter removed is 'ric'.
'rub'+'ric'='RUBRIC'
(Other definitions for rubric that I've seen before include "Authoritative rule of procedure" , "Directions for the conduct of Christian services" , "Document heading" , "Ruddy" , "set of instructions" .)