Zinc or aluminium reveal a reef material (5)
I believe the answer is:
coral
'a reef material' is the definition.
(material making up a coral reef)
'zinc or aluminium reveal' is the wordplay.
'reveal' says the answer is hidden in the clue.
'CORAL' is hidden in the letters of 'zinc or aluminium'.
(Other definitions for coral that I've seen before include "Orangey-pink" , "Marine substance, often forms reef" , "Basic material of South Sea islands" , "Pinkish-orange colour" , "Tiny animal forming reef" .)