He's all eyes, this hard-hearted fellow! (5)
I believe the answer is:
argus
'he's all eyes' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'hard-hearted fellow' is the wordplay.
'hearted' means to look at the middle letters.
'fellow' becomes 'gus' (I am not sure about this - if you are sure you should believe this answer much more).
The middle of 'hard' is 'ar'.
'ar'+'gus'='ARGUS'
'this' acts as a link.
Can you help me to learn more?
(Other definitions for argus that I've seen before include "Giant with one hundred eyes in Greek mythology" , "Many-eyed mythical monster" , "Pheasant, butterfly, a hundred-eyed monster" , "In Greek myth, a many-eyed monster slain by Hermes" , "Monster with many eyes slain by Hermes in Greek myth" .)