Clay, later black, one's used to build defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'clay later black one's' is the wordplay.
'clay later' becomes 'ali' (I've seen this in another clue).
'black' becomes 'b' (abbreviation as in pencil classifications).
'one' becomes 'i' (Roman numeral).
'ali'+'b'+'i'='ALIBI'
'used to build' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Evidence one was not present" , "Evidence one was not at crime scene" , "Accused person's defence" , "Legal defence - was elsewhere" , "Defence against accusation - was seen elsewhere at the time" .)