Dixon Marble scattered eight hits giving up only two runs in seven innings, but was saddled with his second loss of the summer.
St. Joe (29-11, 18-10 MIAA) started the scoring when Brandon Huske reached on an error in the first, and scored on a Joe Koerper groundout.
Clarinda's Justin Smith broke through against Marble in the fourth inning on a sinking line drive to left. It dropped in for a RBI double.
D'Marco Poindexter put the A's on top with a run-scoring single in the seventh.
The Mustangs had chances to score throughout the game. They put runners on base in all but two innings, with their biggest opportunity coming in the bottom of the seventh.
St. Joe loaded the bases with none out and the heart of the order due up, but Trent Hill grounded out, Koerper struck out and Mike Sherburne popped out to end the threat.
The Mustangs stranded a total of 11 runners on base in the game, and their MINK-low batting average dropped to .209.
The team continues its home stand at Phil Welch with a doubleheader Friday evening against Branson.