Material in small opening in marine growth (9)
I believe the answer is:
corporeal
'material' is the definition.
('corporeal' can be a synonym of 'material')
'small opening in marine growth' is the wordplay.
'small opening' becomes 'pore' (I've seen this before).
'in' is an insertion indicator.
'marine growth' becomes 'coral' (I've seen this before).
'pore' going inside 'coral' is 'CORPOREAL'.
'in' is the link.
(Other definitions for corporeal that I've seen before include "physical" , "Bodily" , "pink material" , "not spiritual" , "of flesh" .)