Irregular ultimately out fighting to install Left? (9)

I believe the answer is:

guerrilla

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

'irregular' is the definition.
(I know that irregular can be written as guerrilla)

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

Can you help me to learn more?

(Other definitions for guerrilla that I've seen before include "Politically motivated fighter" , "Partisan" , "Person who fights as one of small independent group" , "Underground fighter" , "Irregular fighter" .)

I've seen this clue in The Independent.

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.