Fat copper initially picking plant (9)
I believe the answer is:
buttercup
'plant' is the definition.
(buttercup is a kind of plant)
'fat copper initially picking' is the wordplay.
'fat' becomes 'butter' (I've seen this before).
'copper' becomes 'cu' (chemical symbol).
'initially' suggests taking the first letters.
The initial letter of 'picking' is 'p'.
'butter'+'cu'+'p'='BUTTERCUP'
(Other definitions for buttercup that I've seen before include "Common yellow meadow plant" , "Common yellow flower" , "Rich yellow wild flower" , "Dockside seller" , "Common meadow flower" .)