Opening in material of reef relating to body (9)
I believe the answer is:
corporeal
'relating to body' is the definition.
(corporeal means relating to someone's body)
'opening in material of reef' is the wordplay.
'opening' becomes 'pore' (pore is a kind of opening).
'in' indicates putting letters inside.
'material of reef' becomes 'coral' (material of a coral reef).
'pore' going into 'coral' is 'CORPOREAL'.
(Other definitions for corporeal that I've seen before include "pink material" , "physical" , "of flesh" , "not spiritual" , "Bodily" .)