Drinking vessels (4)
'drinking vessels' is the definition.
(mug is a kind of drinking vessel)
This is all the clue.
(Other definitions for mugs that I've seen before include "Cups without saucers" , "Attacks" , "fools and > grimaces" , "Robs with the threat of violence" , "Cups; dupes" .)
(Other definitions for beakers that I've seen before include "Flat-bottomed jars" , "Flat-bottomed cups" .)