Break down copper with small crack (8)
I believe the answer is:
dissolve
'break down' is the definition.
(I've seen this before)
'copper with small crack' is the wordplay.
'copper' becomes 'DI' (detective inspector, type of copper).
'with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'crack' becomes 'solve' (crack can mean to solve or decipher).
'di'+'s'+'solve'='DISSOLVE'
(Other definitions for dissolve that I've seen before include "Break up and merge with a liquid" , "melt away" , "Cause to go into a solution" , "Turn into liquid" , "Melt in fluid" .)