Aren't car bits ordered? I forget (12)
I believe the answer is:
scatterbrain
'i forget' is the definition.
(a scatterbrain is a forgetful person)
'aren't car bits ordered?' is the wordplay.
'ordered?' indicates anagramming the letters.
'arent'+'car'+'bits'='arentcarbits'
'arentcarbits' is an anagram of 'SCATTERBRAIN'.
(Other definitions for scatterbrain that I've seen before include "Person unable to think clearly" , "Person incapable of concentration" , "Disorganised thinker" , "One who cannot concentrate" , "Flighty and disorganised person" .)