insert

To insert something is to put it into something else. Into a slot, a gap, or a sentence.

The word is more formal and more precise than put in, which is why instructions and manuals use it: insert the card, insert tab A into slot B. Writing uses it for adding text, so inserting a clause means putting it into an existing sentence. As a noun, stressed on the first syllable, an insert is something placed inside, such as a loose leaflet in a magazine or a shot dropped into a film.