Limits current copper to medium pepper (8)
I believe the answer is:
capsicum
'pepper' is the definition.
(I know that pepper can be written as capsicum)
'limits current copper to medium' is the wordplay.
'limits' becomes 'caps' (cap can mean a limit or maximum).
'current' becomes 'I' (symbol for electric current).
'copper' becomes 'Cu' (chemical symbol).
'to medium' becomes 'm' (as in UK clothing sizes).
'caps'+'i'+'cu'+'m'='CAPSICUM'
(Other definitions for capsicum that I've seen before include "hot stuff perhaps" , "American pepper" , "Eg, chilli pepper" , "Pepper plant" , "Sweet or chilli pepper" .)