"[Tell the believing men to reduce [some] of their vision]", meaning: to lower their gaze from all acts of disobedience. The [min] (from) here is an additive particle (zā'idah).

Yahya [narrated] from Hammad ibn Salamah, from Yunus ibn 'Ubayd, from Abu Zur'ah ibn 'Amr ibn Jarir al-Bajali, from his father, who said: "I asked the Messenger of Allah, peace be upon him, about an accidental glance, and he said: 'Turn your gaze away'."