The glass with the wobbly base? (7)
I believe the answer is:
tumbler
'the glass' is the definition.
(tumbler is a kind of glass)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tumbler that I've seen before include "appears to be falling" , "A drop from a bar may occupy this" , "A glass; part of lock" , "One flips" , "Stemless straight-sided glass" .)