Hi JohnKanak,
Welcome to Affinity Forums
The Clip Canvas option is only available in Affinity Photo. In Designer you have to go to the Document Setup dialog ( menu File ▸ Document Setup...) and change the document dimensions to match the size of the image. Make sure Objects will: Anchor to Page button is selected. Alternatively select all the layers, copy to clipboard then go to menu File ▸ New From Clipboard.
For artboard based documents (not the case here), you can use the Move Tool or the Artboard Tool to drag the boundaries (bounding box handles) of the artboard (make sure the artboard layer is selected in the Layers panel).
