Clough believes his side can and will play better as they try to book a Wembley final.
“I wish the second leg was tomorrow as I think we can play a lot better than we did tonight,” he said.
“I think our nerves were a bit edgy at times and we didn't get our foot on the ball and play. I think we will be better away from home.
“But it was another home victory, which is 14 out of 16, which is incredible considering the pressure we were under in the last half-hour.
“A lot of credit to Northampton after they got the goal – they put it on us and we had to defend extremely well.”
Stags led 2-0 at the break and Clough said: “They were both good goals, good passing and good finishes. Jordan Bowery's was almost a carbon copy of the one he got against Forest Green last week.
“He is in the right position again for a tap-in at the far post and hopefully justified him keeping his place.
“The effort and endeavour was remarkable tonight, as it has been for most of the season. We didn't play anywhere nearly as well as we have done in recent weeks and months, but that's the nature of the play-offs.
“You just have to try to be effective and win the game. Also, don't make mistakes - we made one tonight and were punished for it.
“We said at half-time let's do everything we can to get that third goal and we had two or three opportunities in the first 10 minutes of the second half.
“Unfortunately we conceded not long after that and the game turned.
“We gave the ball away cheaply on the edge of their box and then that's their strength, breaking as they did.
“They did very well at stopping us playing our normal game tonight.
“The minimum we should have had tonight was a 2-0.”
Clough added: “If we didn't know before we now know what Northampton are about and the way that they can play and put you under pressure. It's difficult to play against. That's why they were a few minutes away from getting automatic promotion.
“They are a good team with a lot of League One players from last season when they came down. It's going to be an interesting game on Wednesday night.
“Rhys Oates caused them problems all night and they did well to stick with him at times. I don't think he got many decisions go his way tonight, but then I don't think we did as a team.
“We are still in there after 47 games. We are still in with a chance.”