Material in reef, cold on examination (5)
I believe the answer is:
coral
'material in reef' is the definition.
(as in a coral reef)
'cold on examination' is the wordplay.
'cold' becomes 'c' (eg on taps).
'on' says to put letters next to each other.
'examination' becomes 'oral' (oral is a kind of examination).
'c'+'oral'='CORAL'
(Other definitions for coral that I've seen before include "Tone" , "Reef-forming sea life" , "Tiny animal forming reef" , "Orangey-pink" , "Hard pink substance formed by skeletons of tiny sea creatures" .)