leylandii

The only thing possible is to disguise it - maybe by planting a climber? Or another shrub? Once a Conifer has died on one side, it will not grow new shoots from the old wood. Sorry to be the bearer of bad news.

There is a new disease affecting leyandii leaving it in might affect the others if it is diseased rather than lack of light.

I had a similar problem with a garden that I did recently, I ended up cutting the tree into a standard and removing the top growth. I now can plant under the tree, acid lovers and forest edge plants work well. I also have a clematis and a honeysuckle growing up the trunk.

I have a conifer hedge which had outgrown its space and, like you, I had to reduce it. It has had an ugly bare patch on one side for a year or so but it is gradually being engulfed by the healthy side as it responds to the light. So, if you are happy to wait and give it a chance it may heal itself, but it certainly will not be an instant makeover.