Former copper, extremely sincere, giving pardon (6)
I believe the answer is:
excuse
'pardon' is the definition.
('excuse' can be a synonym of 'pardon')
'former copper extremely sincere' is the wordplay.
'former' becomes 'ex'.
'copper' becomes 'cu' (chemical symbol).
'extremely' says to hollow out the word (remove centre letters).
'sincere' with its centre taken out is 'se'.
'ex'+'cu'+'se'='EXCUSE'
'giving' acts as a link.
(Other definitions for excuse that I've seen before include "Plea; let off" , "Allow to be released from a duty" , "Mitigate; forgive" , "Make apologies for" , "Absolve" .)