One killing animal — a small offence (8)

I believe the answer is:

assassin

I'm a little stuck... Click here to teach me more about this clue!

'one killing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'animal a small offence' is the wordplay.
'animal' becomes 'ass' (term for a donkey).
'small' becomes 's' (abbreviation - e.g. clothes size).
'offence' becomes 'sin' (a sin is an offence or transgression).
'ass'+'a'+'s'+'sin'='ASSASSIN'

Can you help me to learn more?

(Other definitions for assassin that I've seen before include "Political/religious killer" , "Killer of famous person" , "I kill" , "Killer of prominent person" , "Paid killer" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.