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