What does the cryptic crossword clue "Unprompted, son sat up with one, working (11)" mean?

The wordplay uses an anagram. anagram of SON + SAT + UP + ONE, = SPONTANEOUS The answer is SPONTANEOUS.

Unprompted, son sat up with one, working (11)

Daily Mail Cryptic #16425 | 29 Across | 2020-10-15 flower