No way! Preposterous one won't face up to reality (7)
I believe the answer is:
ostrich
'one won't face up to reality' is the definition.
(as in an ostrich with its head in the sand)
'no way preposterous' is the wordplay.
'no' becomes 'o' (resembles 0 - 'no' can mean 'zero').
'way' becomes 'st'.
'preposterous' becomes 'rich' (rich can mean outrageous or unbelievable).
'o'+'st'+'rich'='OSTRICH'
(Other definitions for ostrich that I've seen before include "Fast-running African flightless bird" , "one tries to hide in the sand" , "Chorist (anag.)" , "Large flightless bird of Africa" , "Someone unprepared to face the facts" .)