Defiantly zestful solver quashing that anger (5,2,5)
I believe the answer is:
young at heart
'quashing that anger' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'defiantly zestful solver' is the wordplay.
I cannot really see how this works, but
'solver' could be 'you' (the person solving the crossword) and 'you' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for young at heart that I've seen before include "Elderly, but not feeling it" , "Essentially youthful" , "Old, though not thinking as an oldster" , "Aged but lively" , "Sinatra ballad - with 4d" .)