The better his work is, the greater the strain (8)
I believe the answer is:
composer
This clue might be a double definition.
'the better' is the first definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'his work is the greater the strain' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
Can you help me to learn more?
(Other definitions for composer that I've seen before include "Liszt or Ravel, say" , "Beethoven or Brahms, for instance" , "eg Ravel" , "Person who writes music" , "One creates music" .)