Half rhyme (also called near rhyme or slant rhyme) occurs when words have similar but not identical vowel sounds and ending consonant sounds.
In the case of "cloth" and "wash":
* Vowel sounds: The vowel sounds are completely different (long "o" in "cloth" and short "a" in "wash").
* Consonant sounds: The ending consonant sounds are also different ("th" in "cloth" and "sh" in "wash").
Therefore, "cloth" and "wash" are not half rhymes. They do not share any similar sounds.