Overgrown Staghorn

Hi, 

in our small front garden we have what I'm pretty sure is a sumac (I'll add a picture later on). It came with the house and was already very large. I've recently cleared away a lot of wild growth in the hope that I could make use of the space around the tree but it's so big that nothing seems to be able to flourish under it, especially since the foliage cuts out the sunshine. 

I'd like to make some quite large cuts to seriously reduce the size but I'd rather not kill it - does anyone know if large cuts are possible with this tree and if so the best time to do it. 

Thanks in advance,

James