Limits one 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 one copper to medium' is the wordplay.
'limits' becomes 'caps' (capping is a kind of limiting).
'one' becomes 'i' (Roman numeral).
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'to medium' becomes 'm' (as in UK clothing sizes).
'caps'+'i'+'cu'+'m'='CAPSICUM'
(Other definitions for capsicum that I've seen before include "Sweet or chilli pepper" , "Pepper supplier" , "vegetable?" , "Chili pepper genus" , "Tropical plant producing peppers -- music cap (anag)" .)