Riches loaned in extremes of penury (6)
I believe the answer is:
plenty
'riches' is the definition.
(thesaurus)
'loaned in extremes of penury' is the wordplay.
'loaned' becomes 'lent' ('lend' can be a synonym of 'loan').
'in' means one lot of letters goes inside another.
'extremes of' says to hollow out the word (remove centre letters) (only the letters at the extremes of the word).
'penury' with its middle removed is 'py'.
'lent' going within 'py' is 'PLENTY'.
(Other definitions for plenty that I've seen before include "Copiousness" , "A lot" , "As much as necessary" , "Amplitude or abundance" , "More than enough" .)