long radical (no. 168)
镸 is a radical form meaning 'long'. It is not used as an independent character in modern Japanese, but appears as a component in other kanji to convey the idea of length or elongation.
镸 is a variant form of 長 (long), used as a radical. It likely originated as a simplified or stylized version of 長 for use in compound characters.
镸 looks like a stretched version of 長, with the top strokes simplified. Imagine it as a long measuring tape unrolled to show its length.
For チョウ, picture a long queue of people stretching out like the radical 镸: queue -> チョウ, and the line is very long.