Kiss big copper's behind! (8)
I believe the answer is:
osculate
'kiss' is the definition.
('osculate' can be a synonym of 'kiss')
'big copper's behind' is the wordplay.
'big' becomes 'os'.
'copper' becomes 'cu' (chemical symbol).
'behind' becomes 'late'.
'os'+'cu'+'late'='OSCULATE'
(Other definitions for osculate that I've seen before include "Locate us engaged in pretentious kiss" , "Touch with the lips" , "Kiss in lacteous way" , "Kiss pretentiously?" , "O, cute Sal could kiss" .)