Amerindian's poetic apostrophe to a wise man (5)
I believe the answer is:
osage
'amerindian's' is the definition.
(the Osage Nation are a Native American people)
'poetic apostrophe to a wise man' is the wordplay.
'poetic apostrophe' becomes 'o' (I can't explain this - if you can you should give a lot more credence to this answer).
'to a wise man' becomes 'sage' (sage is a kind of wise man).
'o'+'sage'='OSAGE'
(Other definitions for osage that I've seen before include "Amerindian" , "Member of the Native American people, once occupying the Missouri valley" , "Missouri tribe; go sea (anag.)" , "Native Americans from Missouri - age so (anag)" , "Brave" .)