Founder developing a rich part (9)
I believe the answer is:
patriarch
'founder' is the definition.
(thesaurus)
'developing a rich part' is the wordplay.
'developing' is an anagram indicator (letters developed into a new form).
'a'+'rich'+'part'='arichpart'
'arichpart' is an anagram of 'PATRIARCH'.
(Other definitions for patriarch that I've seen before include "Paterfamilias" , "Male head of family or tribe" , "tribal head" , "Abraham or Isaac, perhaps" , "like Abraham was" .)