Welsh hero’s fierce expression about death (9)
I believe the answer is:
glendower
'welsh hero's' is the definition.
The definition suggests a singular noun which matches the answer.
'fierce expression about death' is the wordplay.
'fierce expression' becomes 'glower' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'death' becomes 'end' (synonyms).
'glower' placed around 'end' is 'GLENDOWER'.
Can you help me to learn more?
(Another definition for glendower that I've seen is " revolting chieftain".)