One handling expert scores, possibly (9)
I believe the answer is:
processor
'one handling expert scores possibly' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'handling expert scores possibly' is the wordplay.
'handling' means one lot of letters goes inside another.
'expert' becomes 'pro' (short for professional).
'possibly' is an anagram indicator.
'scores' is an anagram of 'cessor'.
'pro' going inside 'cessor' is 'PROCESSOR'.
Can you help me to learn more?
(Other definitions for processor that I've seen before include "See 25" , "computer unit" .)