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 am not sure about this - if you are sure you should believe this answer much more).
'upset over' shows that the letters should be reversed in order (knocked over).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'american' becomes 'US' (United States).
'im' reversed gives '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" .)