Concrete is turned over in barrel, big, naturally (8)
I believe the answer is:
tangible
'concrete' is the definition.
('tangible' can be similar in meaning to 'concrete')
'turned over in barrel big naturally' is the wordplay.
'turned over' is a reversal indicator.
'in' indicates a hidden word.
'elbignat' is hidden in the letters of 'barrel big naturally'.
'elbignat' back-to-front is 'TANGIBLE'.
'is' acts as a link.
(Other definitions for tangible that I've seen before include "Clear and definite, can be touched" , "Potentially touched" , "Bleating in clear and definite manner" , "Capable of being touched" , "Touchable, real" .)