Carbon test for marine growth (5)
I believe the answer is:
coral
'marine growth' is the definition.
(I've seen this before)
'carbon test' is the wordplay.
'carbon' becomes 'c' (C is the chemical symbol for carbon).
'test' becomes 'oral' (oral is a type of examination).
'c'+'oral'='CORAL'
'for' is the link.
(Other definitions for coral that I've seen before include "Orangey-pink" , "Pink growth which forms reefs and islands" , "Marine polyps, often forms reefs" , "Creature building reef" , "Marine growth" .)