Possible Clues:

• One might feel spare there, having to keep it
• Available if needed
• Concerning answer, it's available for use if needed
• Put aside for later
• Held, as for a rainy day
• Like rainy-day provisions
• Popular member of the army at one's disposal in case a problem arises
• Held back, perhaps
• Held back playing for second eleven? Not quite
• Kept back popular book