A few lines of preceding context, starting at 364:
Arcadas insuetos acies inferre pedestris
ut vidit Pallas Latio dare terga sequaci
aspera aquis natura loci dimittere quando
suasit equos, unum quod rebus restat egenis, (line 367)
nunc prece, nunc dictis virtutem accendit amaris;
The Arcadians unused to attacking on foot
when Pallas saw them [the Arcadians] running away before pursuing Latins
(the rough nature of the place caused by flowing water persuaded
them to send away the horses) *unum quod rebus restat egenis*
now by pleas now by bitter words he [Pallas] kindled their courage
I can't make out "unum quod rebus restat egenis"
It looks like "insofar as alone in this needful business he (Pallas) stood fast"
The English translation in the Loeb Classical LIbrary gives:
"then as the one hope in such straits"
However, I can't see how they got this.