Yellowish-brown gemstone (5)
I believe the answer is:
topaz
'yellowish-brown gemstone' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for topaz that I've seen before include "White or colourless gemstone" , "Yellow or pale blue gem" , "South American hummingbird" , "Yellow quartz used for gemstones" , "Shade of yellow -- gem" .)