He shouldn't be there - rep asserts otherwise (10)
I believe the answer is:
trespasser
'he shouldn't be there' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'rep asserts otherwise' is the wordplay.
'otherwise' indicates an anagram.
'rep'+'asserts'='repasserts'
'repasserts' anagrammed gives 'TRESPASSER'.
Can you help me to learn more?
(Other definitions for trespasser that I've seen before include "One unlawfully entering another's land" , "Illegal enterer of land" , "one of a number praying for forgiveness?" , "sinner" , "Intruder into another's land" .)