The Bees were well organised in the first-half but the floodgates opened after the break as two goals from Kabongo Tshimanga and strikes from Akwasi Asante and Jeff King handed the Spireites the three points.
Town have lost just once in 21 league matches now.
“The better team won,” Brennan said.
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"They are the best side we have played by an absolute mile in my reign.
"That is the standard it takes to get in the title race in this division.
"I thought they had a very experienced team. We are young and we are learning.
"Their front two, in the end, were devastating, especially in the second-half.
"We were very competitive. The first goal is always going to be key and we had a great chance just before their first goal.
"To be fair to them, what they did, they sensed a kill, and they killed us. That is what good teams do and they are a very good side.
“In the end their front two were too hot to handle.”
He added:"You have got to give them a lot of credit, they have come here and they have done a good job on us.
"I thought we played well in the first-half and I actually thought we started the second-half excellently as well.
"It was competitive, there was nothing in the game up until that first goal.
"We have just got to take our medicine today, they were the much better side.”