Adam Clarke on 1 Corinthians 9:7

Who goeth a warfare - at his own charges? - These questions, which are all supposed from the necessity and propriety of the cases to be answered in the affirmative, tend more forcibly to point out that the common sense of man joins with the providence of God in showing the propriety of every man living by the fruits of his labor. The first question applies particularly to the case of the apostle, τις στρατευεται ιδιοις οψωνιοις· Does a soldier provide his own victuals? Οψωνιον is used to express the military pay or wages, by the Greek writers; for the Roman soldiers were paid not only in money but in victuals; and hence corn was usually distributed among them. See on Luk 3:14 (note).