When you say goal clearance I take it you mean, after the ball has gone out of play? If so then no the 6 second stipulation doesn’t apply to that type of restart. The goal clearance is like any other type of restart and if a goalkeeper was taking too long to get the ball back into play then you (as the ref) would deal with that situation in the same ways as an outfield player taking too long to take a free kick, a kick in or a corner etc. I hope that helps.

Mick, That was the clarity I was looking for. Thank you very much. 

Hope to see you at the Lincoln FA WF Referee's course: 20th May

This may also be worth mentioning in relation to 6 seconds even though it is not a goal clearance.  Under Law 12,An indirect free kick is awarded if a goalkeeper allows the ball to stop in a stationary position in the penalty area without touching it for more than six seconds.