He could give a tail-ender a scar (6)
I believe the answer is:
rascal
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'give a tail-ender a scar' is the wordplay.
'give' indicates an anagram (I've seen 'giving' mean this (the letters give the letters for another word)).
'ender' says to take the final letters (I've seen 'end of' mean this).
The final letter of 'tail' is 'l'.
'a'+'l'+'scar'='alscar'
'alscar' anagrammed gives 'RASCAL'.
'could' is the link.
Can you help me to learn more?
(Other definitions for rascal that I've seen before include "Reprobate" , "Michievous person/child" , "Villain" , "not one to be admired" , "Mischievous person or confused lascar" .)