Doubtful upstart runs off to Evans Head (6)
I believe the answer is:
unsure
'doubtful' is the definition.
(similar in meaning)
'upstart runs off to evans head' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'off' indicates anagramming the letters.
'head' says to take the initial letters.
The first letter of 'evans' is 'e'.
'runs' is an anagram of 'nsur'.
'u'+'nsur'+'e'='UNSURE'
(Other definitions for unsure that I've seen before include "Far from certain" , "Not certain, somewhat confused" , "Not certain or decided" , "self-confidence" , "Not certain, dubious" .)