Alibi of former copper's false, finally (6)
I believe the answer is:
excuse
'alibi' is the definition.
(I've seen this before)
'former copper's false finally' is the wordplay.
'former' becomes 'ex'.
'copper's' becomes 'cus' (chemical symbol).
'finally' indicates one should take the final letters.
The final letter of 'false' is 'e'.
'ex'+'cus'+'e'='EXCUSE'
'of' is the link.
(Other definitions for excuse that I've seen before include "Seek to justify (a fault)" , "Free - apology" , "Pardon or forgive" , "Mitigate; forgive" , "A self-justification" .)