Reduced ambition to join one native group (4)
I believe the answer is:
hopi
'native group' is the definition.
(Native American tribe)
'reduced ambition to join one' is the wordplay.
'reduced ambition' becomes 'hop' (I can't explain this - if you can you should give a lot more credence to this answer).
'to join' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'hop'+'i'='HOPI'
Can you help me to learn more?
(Other definitions for hopi that I've seen before include "Member of a Pueblo Indian people of northeastern Arizona" , "Language" , "Arizonan Pueblo Indian" , "Pueblo tribe member" , "Arizona inhabitant" .)