British Power introducing complete copper plant (9)
I believe the answer is:
buttercup
'plant' is the definition.
(I know that buttercup is a type of flowering plant)
'british power introducing complete copper' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'power' becomes 'p' (symbol in physics).
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'complete' becomes 'utter' (I've seen this before).
'copper' becomes 'cu' (chemical symbol).
'b'+'p'='bp'
'utter'+'cu'='uttercu'
'bp' going around 'uttercu' is 'BUTTERCUP'.
(Other definitions for buttercup that I've seen before include "Weed with rich yellow flowers" , "Dockside seller" , "Rich yellow wild flower" , "Common grassland plant, poisonous to livestock" , "Meadow flower poisonous to cattle" .)