But for his sad heart, he would get ten out of ten! (5)
I believe the answer is:
texan
'but for his sad heart he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'would get ten out of ten' is the wordplay.
'would' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'get' is an insertion indicator.
'out of' indicates putting letters inside (some letters on the outside of others).
'ten' becomes 'X' (Roman numeral).
'ten' placed around 'x' is 'texn'.
'a' put within 'texn' is 'TEXAN'.
Can you help me to learn more?
(Other definitions for texan that I've seen before include "Eg, native of Dallas" , "From a large US state" , "One from SW USA" , "One from south-west USA" , "like LBJ" .)
