The 19-year-old, who impressed for England Under-21s in Israel this summer, is regarded as one of the brightest prospects in the Championship.
"Now what we have had are three bids, two that have been accepted and one that hasn't been accepted. And now it's up to Nathan if he wants to speak to those two clubs and see if it's right for him," Clark told the Birmingham Mail.
"What those other clubs have to do who haven't put bids in yet is get to the level where the other two are at. It's another one where you don't want to lose the player, but you understand the reasons why he has to go.
"Unfortunately that's where we are at the moment and we have just got to get on with it. Nathan was terrific for me second-half of the season. I think he kicked on again from where he had been in his first full season and then he carried it all on in the under-21s. That's why the interest has intensified in him."