Possible Clues:

• Not to be seen at the rainbow's end
• It's invisible below lightly cooked bit of dinner
• Hot objects radiate this
• (Of radiation) with a longer wavelength than light
• Rare find unfortunately can't be seen
• The refrain of D cannot be seen in colour
• Unusual day follows below opposite to 1 across
• Fan drier (anag) - type of radiation
• Radiation band
• Poem on strange duke's taken in new light