One likely to succeed if he hasn't already (9)
I believe the answer is:
inheritor
'one likely to succeed if he hasn't already' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'succeed if he hasn't already' is the wordplay.
I cannot quite see how this works, but
'succeed' could be 'inherit' (I've seen this before) and 'inherit' is found within the answer.
The remaining letters 'or' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for inheritor that I've seen before include "He comes into estate" , "Beneficiary under will" , "Heir" , "One mentioned in will" , "Successor" .)