Possible Clues:

• Low order batsman sent in when a wicket falls near the end of the day's play
• Batsman going up the order gets guard
• Bat expected to hang in there, as light fails
• Vigilant chap who never does a day's work
• Thirteen says when it's his turn to look out
• Nocturnal guard
• Dutch painting, piece showing bat coming out in the gloaming
• Astronomer's security guard?
• Security guard
• Guard of premises not in use