素っ is read as すっ (su) and means most.
すっ
su
しろうと
respectable woman (i.e. not a prostitute, hostess, geisha)
もと
cause
すなお
straight (e.g. hair)
そざい
subject matter
すばやい
quick (understanding, judgement, etc.)
そしつ
nature
すがお
face with no make-up
すばらしい
wonderful
すてき
lovely
そぼく
simple
そりゅうし
elementary particle
そびょう
drawing