الِاقْتِصَادُ is the subject; نِصْفُ الْمَعِيشَةِ is the predicate. The expression فِي النَّفَقَةِ specifies the area of moderation: spending.
فِي is a preposition meaning “in.” نِصْفُ الْمَعِيشَةِ is a possessive construction: نِصْفُ means “half,” and الْمَعِيشَةِ means “living” or “livelihood.” Together: “half of livelihood.”
Do not silently paraphrase نِصْفُ as merely “some part.” At the same time, this is not a numerical spending schedule applicable to each person; the statement strongly emphasizes the importance of moderation.