Spaceman got nr. Mars, going off course (9)

I believe the answer is:

armstrong

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

'spaceman' is the definition.
(I know that Armstrong is a spaceman)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for armstrong that I've seen before include "Jazz legend" , "renowned trumpeter" , "jazz musician" , "See 21 Across" , "Space pioneer" .)

I've seen this clue in The Guardian.

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