Sack dunderhead who isn't married (7)

I believe the answer is:

dismiss

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

'sack' is the definition.
(I know that sack can be written as dismiss)

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

Can you help me to learn more?

(Other definitions for dismiss that I've seen before include "military order" , "Throw out" , "Sack - send away" , "Cease to consider" , "Sack (from job)" .)

I've seen this clue in The Irish 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.