Lubricates drunken serenade, endlessly filling empty glass (7)
I believe the answer is:
greases
'lubricates' is the definition.
('grease' can be a synonym of 'lubricate')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for greases that I've seen before include "Lubricates" , "One of those animal products" , "Bribes" .)