Thief looking at gull from behind (6)

I believe the answer is:

looter

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

'thief' is the definition.
(I know that looter is a type of thief)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for looter that I've seen before include "Stealer of goods" , "Thief during unrest" , "criminal" , "Stealer during riot" , "Wartime stealer" .)

I've seen this clue in The Guardian.

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.