The location states the following :

Any Phase : Kneel Hidden Vale pass to put into play a Clansman character into play from your hand, knelt as an attacker during any challenge you have initiated.

Can i put into play a Clansman character knelt or do i need to kneel a clansman character in play first ?

You need a character to initiate the challenge (it doesn't have to be a Clansman). Once the challenge is initiated, you can use Hidden Vale Pass to add a Clansman in it from your hand (assuming the challenge didn't abort because all characters were removed from it). You can even do it after defenders have been declared and the icons do not matter.

The important thing with Hidden Vale Pass is that you cannot use it to initiate a challenge. You have to initiate the challenge first (which means declaring - and usually kneeling - another character as an attacker) in order to have a challenge in progress that the Clansman from the HVP can join.