Surely being led astray must have fines halved? (2,6)

I believe the answer is:

of course

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

'surely' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for of course that I've seen before include "Obviously!" , "Naturally, certainly" , "About a series of lessons" , "Definitely" , "need I say" .)

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.