Extremely clever, my boss — I owe him (8)
I believe the answer is:
creditor
'i owe him' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'extremely clever my boss' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'my boss' becomes 'editor' (the crossword setter's boss is a newspaper editor).
'clever' with its middle taken out is 'cr'.
'cr'+'editor'='CREDITOR'
Can you help me to learn more?
(Other definitions for creditor that I've seen before include "He's the one owed the money" , "Cried rot (anag.)" , "he advances to you" , "one to whom money is due" , "Moneylender" .)