Get upset, breaking last glass (7)
I believe the answer is:
crystal
'glass' is the definition.
(crystal is a kind of glass)
'get upset breaking last' is the wordplay.
'get upset' becomes 'cry'.
'breaking' indicates an anagram.
'last' with letters rearranged gives 'stal'.
'cry'+'stal'='CRYSTAL'
(Other definitions for crystal that I've seen before include "Quartz, for example" , "Glassware" , "Quartz glass" , "Watch face" , "Type of glass often cut decoratively" .)