It can't run slowly while driving the output shaft. Realistically, most small bent-shaft trimmers can't idle slowly enough to disengage the clutch anyway, though the more expensive and larger machines can. I don't recall you mentioning the brand or model, or posting pictures of it.
We need to know what kind of machine it is, and how fast it has to be going to keep running steadily. If the output shaft is rotating when you pull the starter, though, the clutch is certainly not working, and will have to be fixed. When we see the pictures, we'll try to work out how to dismantle the clutch and find out what is wrong. It sounds as if it may have a broken clutch-spring.
