"The measurement is not very clear."
Translation:La misura è poco chiara.
Tanto and poco can be adverbs, nouns and adjectives: in the first case they're invariable as they don't refer to any noun, in the second case they're male.
- L'acqua chiara è poca: there is little clear water
- L'acqua è un poco chiara: the water is a little clear
- L'acqua è poco chiara: the water isn't very clear
I think "limpida" is more often used as a degree of brightness, as in "a bright light", "a clear day".
In the other hand, "chiara" is often used as a clarity degree of a method or explanation or the way something expressing.
Furthermore, what is wrong with the use of "non è molto chiara" rather than "è poco chiara" is already explained by senior Formica. :)