Using a nightly worked. Here is a better minimal working example that shows 
the effect of the redefined command:

---
title: "Minimal Working Example"
header-includes:
- \let\oldsection\section
- \renewcommand{\section}[1]{\clearpage\oldsection{#1}}
---

# Heading 1 #

Text 1.

# Heading 2 #

Text 2.

The command

pandoc -t latex --latex-engine=xelatex --output=mwe2.pdf mwe2.md

results in a pdf file with each heading on a new page. Producing latex 
output shows the header inclusions inserted without alteration - without 
needing the "backtick" notation John mentioned.

Guess I'm using a pandoc nightly until this behaviour migrates into 
debian/testing :-)

Thanks John, Roland and BP for chipping in to help me.

P.S. It's been ages since I used Google Groups. If there is any way to mark 
this post or thread "solved" please let me know and I will.

On Thursday, 10 August 2017 03:24:41 UTC+9:30, Roland Hieber wrote:
>
> On 08.08.2017 15:56, David Nebauer wrote: 
> > Can't install dev pandoc because the ghc archive downloaded as part of 
> > the process is corrupt. 
>
> You could try installing one of the nightlies from 
> https://github.com/pandoc-extras/pandoc-nightly instead. 
>
>  - Roland 
>

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