I’m so utterly fantastic on many levels! (11)
I believe the answer is:
multistorey
'on many levels' is the definition.
(on multiple storeys or levels)
'i'm so utterly fantastic' is the wordplay.
'fantastic' is an anagram indicator.
'im'+'so'+'utterly'='imsoutterly'
'imsoutterly' anagrammed gives 'MULTISTOREY'.
(Other definitions for multistorey that I've seen before include "Type of car park" , "Built on several levels" .)