November birthstone (5)
I believe the answer is:
'november birthstone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for topaz that I've seen before include "Yellowish-brown gemstone" , "Shade of dark yellow" , "Yellow quartz used for gemstones" , "Silicate gemstone" , "Precious stone - American hummingbird" .)