I confess on radio to mobster’s killing (5)

I believe the answer is:

icing

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

'killing' is the definition.
(ice can informally mean to assassinate)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for icing that I've seen before include "Sugar preparation" , "Sugar-coating" , "Sweet coating on cake" , "You could see this on a cake" , "effect of cold weather" .)

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.