Be obviously upset over the last broken glass (7)
I believe the answer is:
crystal
'glass' is the definition.
(I know that crystal is a type of natural glass)
'be obviously upset over the last broken' is the wordplay.
'be obviously upset over' becomes 'cry' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'broken' indicates an anagram.
'last' anagrammed gives 'stal'.
'cry'+'stal'='CRYSTAL'
Can you help me to learn more?
(Other definitions for crystal that I've seen before include "Very clear glass" , "Quality glass" , "Quartz, for example" , "Clear material such as quartz" , "Glass made at Waterford" .)