Hare-brained puss in pigpen (6)
I believe the answer is:
scatty
'hare-brained' is the definition.
(I've seen this before)
'puss in pigpen' is the wordplay.
'puss' becomes 'cat' (I've seen this in other clues).
'in' indicates putting letters inside.
'pigpen' becomes 'sty' (synonyms).
'cat' going within 'sty' is 'SCATTY'.
(Other definitions for scatty that I've seen before include "Informally silly or crazy" , "Slightly eccentric" , "Slightly crazy" , "Disorganised, unpredictable" , "dizzy" .)