Unconvinced by upstart nurse's organisation (6)
I believe the answer is:
unsure
'unconvinced' is the definition.
(thesaurus)
'by upstart nurse's organisation' is the wordplay.
'by' is a charade indicator (letters next to each other).
'upstart' becomes 'u' (start of the word 'up').
'organisation' is an anagram indicator.
'nurse' is an anagram of 'nsure'.
'u'+'nsure' is 'UNSURE'.
(Other definitions for unsure that I've seen before include "Not certain or confident" , "Far from certain" , "Not certain, dubious" , "Diffident, uncertain" , "regarded as somewhat precarious" .)