"Jane" wrote in message

I've got a lovely, large old cactus that got a little bit damaged
when I
recently moved house. One of its many 'arms' (wonder what they are
called? It really does look like it's spreading its arms out to take
a
bow) got damaged near the 'trunk' of the plant and I had to cut it
off.
I trimmed it down a bit and put it in a little bit of water and I
was
wondering if it would continue to grow, if I should put it in cactus
compost or something. Any ideas?

Take it out of the water, put it somewhere where the wound can dry out
and heal over a few days.
When the wound is dry, then put it into some sandy potting mix water
and wait - keep it dryish rather than wettish.