That fellow, fellow with Peter, is downhearted (10)
I believe the answer is:
chapfallen
'is downhearted' is the definition.
(thesaurus)
'that fellow fellow with' is the wordplay.
'that fellow' becomes 'chap' (both can mean a man).
'fellow with' becomes 'fallen' (I am not sure about this - if you are sure you should believe this answer much more).
'chap'+'fallen'='CHAPFALLEN'
'peter' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for chapfallen that I've seen before include "Dispirited" , "Dejected" .)