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' is a reversal indicator.
'some guests' becomes 'gue' (I can't justify this - if you can 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" .)