。
Clarity is what matters most.
Literal
Clarity [subject-が] most important [polite-です].
The suffix ~さ turns an adjective into an abstract noun: 明確 (clear, な-adjective) → 明確さ ('clarity'). This is the productive -ness/-ity suffix in Japanese. 一番 ('number one, most') + 大事 ('important') is a common superlative.