Poor school rings out each time for a progress-checker (5,3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

[ “broke”(poor/having run out of money) + “pod”(a school/herd of marine animals such as whales) ] replacing ( … out each … for … ) “o”(letter shaped like a ring) with “ A ”. Defn: That which checks/slows down the progress/movement of a vehicle