There's someone looking on again and he's critical (8)
I believe the answer is:
reviewer
'there's someone looking on again and he's critical' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'again and he's critical' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'and' is a charade indicator (letters next to each other).
'he's critical' becomes 'viewer' (I can't justify this - if you can you should give a lot more credence to this answer).
're'+'viewer'='REVIEWER'
Can you help me to learn more?
(Other definitions for reviewer that I've seen before include "critical sort" , "Writer who assesses the merits of a book, play etc." , "Writer on films, books, etc" , "Critic" , "Writer of criticism" .)