My one source of wealth (5)
I believe the answer is:
money
'wealth' is the definition.
(I know that wealth can be written as money)
'my one source of' is the wordplay.
'source of' indicates anagramming the letters.
'my'+'one'='myone'
'myone' anagrammed gives 'MONEY'.
(Other definitions for money that I've seen before include "The 6 [NECESSARY]" , "Lucre" , "Legal tender" , "Bread" , "'A fool and his.....are soon parted'" .)