Glass pigeon (7)
I believe the answer is:
tumbler
I believe this is a double definition.
'glass' is the first definition.
(tumbler is a kind of glass)
'pigeon' is the second definition.
(tumbler is a kind of pigeon)
(Other definitions for tumbler that I've seen before include "One repeatedly turning over" , "Gymnast with a glass" , "appears to be falling" , "A glass; part of lock" , "Constantly dropping" .)