Assertive man I don't upset (8)
I believe the answer is:
dominant
'assertive' is the definition.
(I've seen this before)
'man i don't upset' is the wordplay.
'upset' indicates anagramming the letters.
'man'+'i'+'dont'='manidont'
'manidont' with letters rearranged gives 'DOMINANT'.
(Other definitions for dominant that I've seen before include "Assertive" , "note?" , "Like a boss" , "alpha male?" , "Authoritative" .)