In retrospect, number is up for invested wealth (9)
I believe the answer is:
profusion
'wealth' is the definition.
(thesaurus)
'in retrospect number is up for invested' is the wordplay.
'in retrospect' is a reversal indicator.
'number' becomes 'no' (abbreviation for 'number').
'invested' is an insertion indicator.
'no'+'is'+'up'='noisup'
'noisup' placed around 'for' is 'noisuforp'.
'noisuforp' reversed gives 'PROFUSION'.
(Other definitions for profusion that I've seen before include "Lots" , "Large number" , "Plenty" , "Abundance" .)