1. canvas/canvass The Oxford English Dictionary and Merriam-Webster Dictionary both show the spelling canvas as a variant spelling of the verb “to canvass,” but Chicago Manual of Style, AP Stylebook and Paul Brians’ Common Errors in English Usage agree that the verb meaning “to survey” is spelled with a double s. The spelling canvas is a noun signifying a heavy cloth.
2. clench/clinch Although both words share an origin, in modern usage they are not interchangeable. You clench your fist or teeth, but clinch a deal or a victory. In boxing, “to clinch” means “to grapple at close quarters.” As a noun, clinch is used colloquially to mean “an embrace.”
3. compliment/complement Both words may be used as either nouns or verbs. A compliment is a praising remark; a complement is something that enhances or completes. “To compliment” is to praise”; “to complement” is “to complete.”