He's willing to let others succeed (8)
I believe the answer is:
testator
'he's willing' is the definition.
(I've seen this before)
'to let others succeed' is the wordplay.
I cannot really see how this works, but
'to' is found in the answer.
This explanation may well be incorrect...
(Other definitions for testator that I've seen before include "One willing" , "A man who has made a will" , "He's willing" , "Will signatory" , "Will provider" .)