Significant island wrapped by Christo somehow! (8)
I believe the answer is:
historic
'significant' is the definition.
(of historic importance)
'island wrapped by christo somehow' is the wordplay.
'island' becomes 'i'.
'wrapped by' is an insertion indicator (inserted letters get wrapped up).
'somehow' indicates anagramming the letters.
'christo' with letters rearranged gives 'historc'.
'i' placed inside 'historc' is 'HISTORIC'.
(Other definitions for historic that I've seen before include "Momentous, noteworthy" , "Significant" , "Landmark; famous" , "Epoch-making" , "Connecting with old times" .)