Jerome's knowledge was about right (4)
I believe the answer is:
kern
'jerome's' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'knowledge was about right' is the wordplay.
I cannot quite see how this works, but
'right' could be 'r' (abbreviation) and 'r' is found in the answer.
'knowledge' could be 'ken' (ken is a kind of knowledge) and 'ken' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for kern that I've seen before include "See 14 Across" , "Popular composer" , "See 12" , "Jerome __ , Show Boat composer, d. 1945" , "Old soldier" .)