How to crop a single object not the whole image?

Hi Guys

My first time here.

I have 3 objects in my image. 

I'm familiar with the crop tool which applies to the whole image.

But I want to trim or crop just one of the objects within the image. I'm sure this must be possible but I haven't found the right tool.

I hope I've described this clearly.

If someone could point me in the right direction I'd be very grateful.

Thanks for your time.

B R