Base missing from stone grate (3)
I believe the answer is:
rub
'grate' is the definition.
(I know that grate is a more specific form of the action rub)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for rub that I've seen before include "Ointment for skin" , "Abrade" , "Ay, there's the - (Hamlet)" , "Ointment; chief difficulty" , "Central difficulty" .)