Stone's anger worsened over time (6)
I believe the answer is:
garnet
'stone's' is the definition.
(precious stone)
'anger worsened over time' is the wordplay.
I cannot really see how this works, but
an anagram of 'anger' is 'garne' which is located in the answer.
'time' could be 't' (abbreviation) and 't' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for garnet that I've seen before include "January birthstone" , "Hard glassy mineral used as a gemstone" , "Glassy red, yellow or green gemstone" , "Semi-precious stone of argent type" , "Semi-precious stone of deep transparent red" .)