The Scotland boss is only permitted to name 12 substitutes, meaning that a trio from his tight-knit pool of 26 will be left unstripped in the stand, unable to take part.
Clarke has almost settled on his starting XI, with only one position causing him debate, as Scotland look to make the best possible start to their group campaign that also includes Croatia and England, who face each other on Sunday afternoon at Wembley. However, he admits to having far more trouble deciding which players to leave out altogether.
“I've picked my team except for one position,” said Clarke, “but I'm pretty sure which way I'll go with it.
“The players won't know until the day of the game. “But the more perceptive ones might work it out.
“The way I've worked is that I tell them in the morning. That gives them the afternoon to rest in their rooms and prepare mentally for it.
“For the ones who aren't selected, they prepare to be ready to be part of the squad. So everyone is together when we hit the dressing-room.
“That's worked quite well for us so far so I don't see any reason to deviate from that.
“The hardest part is picking the three who don't make the bench.
“I try to explain to them that it's easy picking the XI. Then I have to pick a bench.
“The ones who are disappointed to be subs have to realise that there are three sitting in the stand. Those guys are the ones I feel more sorry for.
"But I explained right at the start of this process, we've got 26 and everyone will have a different role within the squad.
“They have to understand what that role is and bring their best side to that role.
“They don't know what it is yet but if they bring their best, we've got a chance of being successful.”