Get gradually narrower (5)
I believe the answer is:
taper
'get gradually narrower' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for taper that I've seen before include "Waxed spill" , "Gradually reduce - spill" , "Prate (anag.)" , "Get thinner - lighter" , "Slender candle" .)