Nothing trivial, perhaps, upset tribal leader (8)
I believe the answer is:
geronimo
'tribal leader' is the definition.
'geronimo' can be an answer for 'leader' (I have seen 'Brave leader' mean 'geronimo' so perhaps 'leader' could also mean 'geronimo'). I'm not certain of the 'tribal' bit.
'nothing trivial perhaps upset' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'trivial' becomes 'minor' (I've seen this before).
'perhaps' becomes 'eg' (for example).
'upset' says the letters should be written in reverse.
'o'+'minor'+'eg'='ominoreg'
'ominoreg' reversed gives 'GERONIMO'.
Can you help me to learn more?
(Other definitions for geronimo that I've seen before include "Cry of exhilaration" , "Whee!; Apache chief" , "exhilarated cry" , "war-cry" , "helicopter paratroops' departure call?" .)