A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 21:1The pious are led by the Psalmist to celebrate God's favor to the king in the already conferred and in prospective victories. The doxology added may relate to both Psalms; the preceding of petition, chiefly this of thanksgiving, ascribing honor to God for His display of grace and power to His Church in all ages, not only under David, but also under his last greatest successor, "the King of the Jews." (Psa 21:1-13)
thy strength . . . thy salvation--as supplied by Thee.
psa 21:2The sentiment affirmed in the first clause is reaffirmed by the negation of its opposite in the second.
psa 21:3preventest--literally, "to meet here in good sense," or "friendship" (Psa 59:10; compare opposite, Psa 17:13).
blessings of goodness--which confer happiness.
crown of pure gold--a figure for the highest royal prosperity.
psa 21:4(Compare Sa2 7:13-16). The glory and blessedness of the king as head of his line, including Christ, as well as in being God's specially selected servant, exceeded that of all others.
psa 21:6made him most blessed--or set him "to be blessings," as Abraham (Gen 12:2).
with thy countenance--by sight of thee (Psa 16:11), or by Thy favor expressed by the light of Thy countenance (Num 6:25), or both.
psa 21:7The mediate cause is the king's faith, the efficient, God's mercy.
psa 21:8The address is now made to the king.
hand--denotes power, and
right hand--a more active and efficient degree of its exercise.
find out--reach, lay hold of, indicating success in pursuit of his enemies.
psa 21:9The king is only God's agent.
anger--literally, "face," as appearing against them.
as a fiery oven--as in it.
psa 21:10fruit--children (Psa 37:25; Hos 9:16).
psa 21:11This terrible overthrow, reaching to posterity, is due to their crimes (Exo 20:5-6).
psa 21:12turn their back--literally, "place them [as to the] shoulder."
against the face of them--The shooting against their faces would cause them to turn their backs in flight.
psa 21:13The glory of all is ascribable to God alone.