Becomes narrower (6)
I believe the answer is:
'becomes narrower' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tapers that I've seen before include "Long thin candles" , "Diminishes gradually" , "Narrows towards the end, like a candle" , "Repast with slender candles" , "Makes or becomes gradually narrower" .)