Out of the blue, but a red pencil is incorrect (13)
I believe the answer is:
unpredictable
'out of the blue' is the definition.
(thesaurus)
'but a red pencil is incorrect' is the wordplay.
'is incorrect' indicates an anagram.
'but'+'a'+'red'+'pencil'='butaredpencil'
'butaredpencil' with letters rearranged gives 'UNPREDICTABLE'.
(Other definitions for unpredictable that I've seen before include "erratic" , "Changeable" , "Capricious" .)