Possible Clues:

• 'So long as it's half a decade (4,5)'
• 'Time to be half-decayed, by the sound of it (4,5)'
• Time for only half the decade
• Time will give a lustre to them
• Time for 9 across
• You can get such lustre only through such time.
• Lustre that might be, in short, in the slump