سَعْيًا فِي الْخَيْرِ
Strive in doing good.
Here “سَعْيًا” is a verbal noun (maṣdar) used in place of an imperative verb. The sense presupposes a verb such as “اسْعَ سَعْيًا فِي الْخَيْرِ.”
سَعْيًا: an accusative verbal noun representing the omitted governing verb.
فِي الْخَيْرِ: a prepositional phrase specifying the field of striving.
This construction foregrounds the action itself. In context it should not be reduced to a fragmentary nominal description: the intended force is an exhortation to act.