Stone with coloured bands (5)
I believe the answer is:
'stone with coloured bands' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for agate that I've seen before include "Semi-transparent stone" , "Striped stone" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "Banded ornamental stone" , "marble" .)