Grow thinner and lighter (5)
I believe the answer is:
taper
'lighter' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for taper that I've seen before include "Wax light; narrow" , "Wick; gradually lessen" , "Waxed spill" , "Become narrow" , "Narrow gradually as candle may" .)