Am a little upset first, taking aim (8)
I believe the answer is:
ambition
'aim' is the definition.
(thesaurus)
'am a little upset first' is the wordplay.
'a little' becomes 'bit' (a small bit).
'upset' is a reversal indicator.
'first' becomes 'noi' (resembles No. 1).
'noi' written backwards gives 'ion'.
'am'+'bit'+'ion'='AMBITION'
'taking' acts as a link.
(Other definitions for ambition that I've seen before include "Desire to succeed in life" , "Cherished aspiration" , "wanting to succeed" , "A determination to succeed" , "Objective" .)