It's what let Icarus down (3)

I believe the answer is:

wax

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'down' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for wax that I've seen before include "Increase in strength or size, like ear matter" , "Increase in phase (as moon)" , "Candle material" , "Increase (as moon's phase)" , "Cere - grow larger" .)

I've seen this clue in the Evening Standard.

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