Brightly colored rock (5)
I believe the answer is:
agate
'brightly colored rock' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for agate that I've seen before include "Variegated chalcedony" , "rock with bands" , "Old drama critic" , "Onyx" , "Ornamental form of quartz" .)