Author is upset over new American bar (5)
I believe the answer is:
minus
'bar' is the definition.
(the minus symbol)
'author is upset over new american' is the wordplay.
'author is' becomes 'im' (I can't explain this - if you can you should believe this answer much more).
'upset over' says the letters should be written backwards (knocked over).
'new' becomes 'n' (abbreviation as in New Testament NT).
'american' becomes 'US' (United States).
'im' in reverse letter order is 'mi'.
'mi'+'n'+'us'='MINUS'
Can you help me to learn more?
(Other definitions for minus that I've seen before include "Reduced by the subtraction of - thus?" , "'Take away' sign" , "Less, without" , "Negative arithmetic operation" , "deficiency" .)