In order to understand the meaning of this, we need to have the full context, which would have been obvious to a contemporary reader but is perhaps not today.
The setting in this verse is a school cricket match played at Clifton College school, a leading public (aka private) school in England. We can tell the location from the reference to "the Close" which is a sports ground in Bristol1.
The team playing is ten runs down ("Ten to make and the match to win") with an hour left to play and the next batsman to be out will end the match ("An hour to play and the last man in").
10 runs in an hour is entirely achievable, but the conditions are extremely difficult for a batting team due to the rough pitch and the low level of the sun in the sky ("A bumping pitch and a blinding light").
A game which will have taken most of the day now depends on the performance of a single batting pair - who are generally the weakest in the team. It will have been a tense moment.
In the context of cricket "playing up" has two meanings.
The first, is to play at a level beyond your typical level; for instance an 11 year old playing with 13 year olds would be "playing up".
The second, which is what I think this is a reference to, is often called "playing the ball on the up". There's an extensive discussion of this here but the summary is that this refers to "making contact with the ball before it reaches the top of the bounce" which is. This is a stroke which is , in the words of Bob Woolmer's Art and Science of Cricket "risky, but to the well-set batsman on a good pitch it can pay excellent dividends" (p.138).
In short, the captain here is telling his player to play to the best of his ability and play boldly. He's telling him to be brave.