Crossword Genius

One working on own initiative, describing letter S maybe? (4-7)

Ross

I believe the answer is:

self-starter

'one working on own initiative' is the definition.
(someone who works unsupervised)

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

(Other definitions for self-starter that I've seen before include "Someone with initiative and drive" , "Internally motivated person" , "One who acts on their own initiative" , "Highly-motivated person" .)

I've seen this clue in The Times.
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