Pass plant, being extravagant purchaser (11)
I believe the answer is:
spendthrift
'extravagant purchaser' is the definition.
(someone who spends extravagantly)
'pass plant' is the wordplay.
'pass' becomes 'spend' (synonyms).
'plant' becomes 'thrift' (type of plant).
'spend'+'thrift'='SPENDTHRIFT'
'being' acts as a link.
(Other definitions for spendthrift that I've seen before include "Extravagant" , "Irresponsible spender" , "Irresponsible waster of money" , "Wastrel" , "Extravagantly spending person" .)