Defence is behind in concrete case (8)
I believe the answer is:
rebuttal
'defence' is the definition.
(rebuttal is a kind of defence)
'behind in concrete case' is the wordplay.
'behind' becomes 'butt' ('butt' can be a synonym of 'behind').
'in' means one lot of letters goes inside another.
'concrete case' becomes 'real' (I can't explain this - if you can you should believe this answer much more).
'butt' placed into 'real' is 'REBUTTAL'.
'is' is the link.
(Other definitions for rebuttal that I've seen before include "statement delivered to counter" , "reply to accusation" , "resistance to attack" , "Negation" , "arguing to the contrary" .)