He's apt to offend or upset some guests (5)
I believe the answer is:
rogue
'he's apt to offend' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'or upset some guests' is the wordplay.
'upset' says the letters should be written in reverse.
'some guests' becomes 'gue' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'or' back-to-front is 'ro'.
'ro'+'gue'='ROGUE'
Can you help me to learn more?
(Other definitions for rogue that I've seen before include "Blackguard" , "A dishonest or unprincipled person" , "Scally" , "One dishonest" , "Rascal or reprobate" .)