There’s no confirmation as to Clark’s role, and this is entirely speculation on my part… but he certainly does bear a striking (ahem) resemblance to Brian Cox, who portrayed X2 villain Stryker — the man behind Logan’s shady paramilitary past and adamantium skeleton.
Fox has been fairly silent about the movie so far — we know that Jackman is obviously returning, and that Liev Schreiber has a major role (likely as Victor Creed/Sabretooth). Several actresses were reportedly in the running as Wolvie’s former lover Kayla Silverfox, but no official announcements have been made regarding the cast. So we’ll just keep guessing. (FYI — part of Wolverine’s healing ability is decelerated aging, if you’re wondering why Jackman can still play the character thirty years “younger”.)
Clark may be a relative unknown for such a key part in a big studio feature, but bear in mind that nobody knew Jackman before he grabbed his role as the cranky claw-fisted mutant. (Also note that the studio likely needs to have a certain percentage of homegrown talent on screen in order to qualify for budget-relieving tax incentives gained from shooting in Australia.)