Individual against replacing detective's building material (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'individual against replacing detective's' is the wordplay.
I cannot really understand how this works, but
'against' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for concrete that I've seen before include "Strong, hard, building material" , "Actual - stuff made with cement" , "Substantial, real" , "Firm" , "factual type of evidence?" .)