You can always get a drink out of them! (4)
I believe the answer is:
mugs
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for mugs that I've seen before include "Cups without saucers" , "Gullible people" , "Faces (sl.)" , "Robs with the threat of violence" , "Cups; dupes" .)