What does the cryptic crossword clue "Spaceman got nr. Mars, going off course (9)" mean?

The definition is "Spaceman". The wordplay uses an anagram. anagram of GOT + NR. + MARS, = ARMSTRONG; definition: "Spaceman" The answer is ARMSTRONG.

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

Guardian Cryptic #29289 | 9 Across | 2024-01-26 flower