Related Clues:

• Cool -- cucumber
• Mellow, perhaps
• Retirement factor
• Driver's license stat
• Fit -- fiddle
• Delicate subject, to some
• Mature, as cheese
• Kind of discrimination
• Application form box
• Thick-brick center
• Solid-rock center?
• __ of Reason
• Dead ___ dodo
• Historical chapter
• Thick-brick link