A graduate accountant’s banana plant (5)
I believe the answer is:
abaca
'banana plant' is the definition.
(I've seen this in another clue)
'a graduate accountant's' is the wordplay.
'graduate' becomes 'BA' (someone with a Bachelor of Arts degree).
'accountant' becomes 'ca' (abbreviation for chartered accountant).
'a'+'ba'+'ca'='ABACA'
(Other definitions for abaca that I've seen before include "plant fibre" , "Manila hemp" , "Hemp plant" .)