One rants madly, becoming rich (8)
I believe the answer is:
resonant
'rich' is the definition.
(thesaurus)
'one rants madly' is the wordplay.
'madly' is an anagram indicator.
'one'+'rants'='onerants'
'onerants' with letters rearranged gives 'RESONANT'.
'becoming' acts as a link.
(Other definitions for resonant that I've seen before include "producing vibrant sound" , "(Of sound) deep, clear and continuing to ring" , "Deeply sounding" , "Reverberating" , "Tending to prolong sound, echoing" .)