The lone holdout juror in Lindsay Clancy's murder trial is doubling down on his belief she's guilty ... claiming he had plenty of evidence to back up his position, but he's keeping some of it under wraps.

Michael Desronvil sat down with Fox News's Kayleigh McEnany ... insisting he had absolutely no reasonable doubt about Clancy's guilt, despite 11 other jurors believing she wasn't criminally responsible for killing her 3 children.

And while Desronvil says he had plenty of evidence to support his decision, he's not spilling all the details ... saying he wants to respect the other jurors and let the court process play out.

He does admit things got heated inside the jury room ... which probably isn't surprising, considering he was the only one standing in the way of a unanimous verdict after 38 hours of deliberations.

As TMZ previously reported ... Desronvil already pushed back on accusations he ignored reasonable doubt, telling us last month he believed Clancy carefully planned the killings.

Other jurors have painted a very different picture ... accusing Desronvil of refusing to budge despite allegedly acknowledging doubts about the case.

Clancy's defense maintains she was suffering from postpartum psychosis when her children died ... while prosecutors argued she knew exactly what she was doing.

The case remains up in the air after a judge rejected Clancy's attempt to have the murder charges tossed earlier this month.

Desronvil's full interview airs Thursday morning on "Fox & Friends."