Glass acrobat (7)
I believe the answer is:
tumbler
I believe this is a double definition.
'glass' is the first definition.
(I know that tumbler is a type of glass)
'acrobat' is the second definition.
(synonyms)
(Other definitions for tumbler that I've seen before include "will hold water" , "Glass for whisky" , "Gymnast with a glass" , "A glass; part of lock" , "Constantly dropping" .)