Disorganised; silly (14)
I believe the answer is:
scatterbrained
This clue is a double definition.
'disorganised' is the first definition.
(I've seen this before)
'silly' is the second definition.
(I've seen this before)
(Other definitions for scatterbrained that I've seen before include "Inclined to forget" , "Unfocused" , "Disorganised, lacking concentration" , "absent-minded" , "easily distracted - with 20d" .)