Hopeful undertakings at outset lead to upset (6)
I believe the answer is:
upbeat
'hopeful' is the definition.
(thesaurus)
'undertakings at outset lead to upset' is the wordplay.
'at outset' suggests taking the first letters.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'to upset' becomes 'eat' (eat can mean to upset or distress).
The initial letter of 'undertakings' is 'u'.
'u'+'pb'+'eat'='UPBEAT'
(Other definitions for upbeat that I've seen before include "Optimistic - anacrusis" , "maestro communicates this to orchestra" , "Promising" , "Optimistic (colloq.)" , "Optimistic, positive" .)