Grasses on European copper (5)
I believe the answer is:
reeds
'grasses' is the definition.
(type of grass plant)
'on european copper' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'european' becomes 'e' (abbreviation e.g. EU).
'copper' becomes 'DS' (detective sergeant).
're'+'e'+'ds'='REEDS'
(Other definitions for reeds that I've seen before include "Marsh plants with tall straight stems" , "Thatching stalks" , "Thin pieces of cane in musical instruments" , "River grasses" , "Tall grasses with hollow stems" .)