Apology from retired copper on case of sabotage (6)
I believe the answer is:
excuse
'apology' is the definition.
(I know that apology can be written as excuse)
'retired copper on case of sabotage' is the wordplay.
'retired' becomes 'ex' (prefix meaning former or onetime).
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'on' means one lot of letters go next to another.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'sabotage' with its middle removed is 'se'.
'ex'+'cu'+'se'='EXCUSE'
'from' acts as a link.
(Other definitions for excuse that I've seen before include "A defence for bad behaviour" , "Upstanding community members" , "Make apologies for" , "Absolve" , "A self-justification" .)