Irregular mostly out 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.
('guerrilla' can be a synonym of 'irregular')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for guerrilla that I've seen before include "Irregular bandit" , "Person who fights as one of small independent group" , "Resistance fighter" , "Such tactics" , "freedom fighter?" .)

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.