Fat copper quietly getting plant (9)
I believe the answer is:
buttercup
'plant' is the definition.
(buttercup is a kind of plant)
'fat copper quietly getting' is the wordplay.
'fat' becomes 'butter' (I've seen this before).
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'quietly' becomes 'p' (p for piano and quiet).
'getting' is a charade indicator (letters next to each other).
'butter'+'cu'+'p'='BUTTERCUP'
(Other definitions for buttercup that I've seen before include "Common yellow meadow plant" , "Weed with rich yellow flowers" , "Flowering plant" , "Bright yellow flower" , "growth in the field" .)