Letter of substance reported generosity of spirit (8)
I believe the answer is:
largesse
'generosity of spirit' is the definition.
'letter of substance reported' is the wordplay.
I cannot really see how this works, but
'letter' could be 'ess' (the letter S) and 'ess' is found within the answer.
'substance' could be 'la' (La is an example) and 'la' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for largesse that I've seen before include "Magnanimity, munificence" , "Eelgrass (anag.)" , "Generosity in bestowing gifts" , "Generosity in gift-giving" , "Liberal gift" .)