Identical pixie recreating a mess outside (8)
I believe the answer is:
selfsame
'identical' is the definition.
(synonyms)
'pixie recreating a mess outside' is the wordplay.
'pixie' becomes 'elf' (synonyms).
'recreating' indicates an anagram (I've seen 'recreated' mean this).
'outside' is an insertion indicator (some letters go outside others).
'a'+'mess'='amess'
'amess' is an anagram of 'ssame'.
'elf' going within 'ssame' is 'SELFSAME'.
(Other definitions for selfsame that I've seen before include "Exactly identical" , "The identical kind, not changed or different" .)