Copper put on special point (4)
I believe the answer is:
cusp
'point' is the definition.
(I know that cusp is a type of point)
'copper put on special' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'put on' is a charade indicator (letters next to each other).
'special' becomes 'sp' (abbreviation).
'cu'+'sp'='CUSP'
(Other definitions for cusp that I've seen before include "Pointed end - division between signs of the zodiac" , "Pointed end where two curves meet" , "Point formed by two intersecting curves - point of transition" , "Pointed end (e.g. on tooth)" , "Turning point on which important developments depend" .)