An agreeable chap (3-3)
I believe the answer is:
yes-man
'an agreeable chap' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's an association between them I don't understand?
'agreeable chap' is the wordplay.
'agreeable' becomes 'yes' (I can't explain this - if you can you should believe this answer much more).
'chap' becomes 'man' (chap can mean a man).
'yes'+'man'='YES-MAN'
Can you help me to learn more?
(Other definitions for yes-man that I've seen before include "Flunky" , "He never dissents" , "Sycophant" .)