One who foils trespassers? (6)
I believe the answer is:
fencer
'one who foils trespassers?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for fencer that I've seen before include "Maker of barriers; user of foil" , "One using foil, epee" , "secures a boundary" , "Sporty type" , "A sportsman with swords" .)