He succeeds becoming hornier – it's fantastic! (9)
I believe the answer is:
inheritor
'he succeeds' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'hornier it's fantastic' is the wordplay.
'fantastic' indicates an anagram (a fantastic or improbable ordering of the letters).
'hornier'+'it'='hornierit'
'hornierit' with letters rearranged gives 'INHERITOR'.
'becoming' acts as a link.
Can you help me to learn more?
(Other definitions for inheritor that I've seen before include "One mentioned in will" , "Successor" , "He comes into estate" , "Heir" , "Beneficiary under will" .)