The Saddlers dominated the first-half at Meadow Lane and deservedly went in at the break ahead thanks to a goal from Dan Scarr.
But both teams made a raft of changes on the hour mark with the Saddlers swapping eight players and County six.
And from that point on the hosts seized the advantage with Enzio Boldewijn netting a brace.
"I thought the first 60 minutes were brilliant," Clarke said.
"I thought we were really good. We were on the front foot and should have had two or three goals.
"The last 30 minutes, s***e.
"It was not good enough. We have got some young lads out here but they have got to learn quickly. It is as simple as that.
"I’m not going to gloss over the last 30 minutes.
"But the first 60 I was really pleased with what I saw. We were high energy, we forced them into errors.
"We have got to be more clinical, yes, we understand that. But a lot of the things we have been working on, the high press, worked on numerous occasions in that first 60. So I am going to focus on that at the minute."
Striker's Josh Gordon and Elijah Adebayo both looked lively throughout.
But neither of them were able to take any of the opportunities that came their way.
"We got into dangerous positions," Clarke added.
"Josh and Elijah worked really hard up there but they have got to be clinical in those key moments and at key times in games.
"We didn’t do that. We didn’t get that second or even third goal to see it off.
"Then there were wholesale changes after 60 minutes and we didn’t perform to the standards I expect.
"But for 60 minutes we were pretty dominant which is pleasing to see."