Yellowish-brown gemstone (5)
I believe the answer is:
topaz
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">yellowish-brown gemstone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)