Ostrich one flying? That's momentous (8)
I believe the answer is:
historic
'momentous' is the definition.
(I've seen this before)
'ostrich one flying?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'flying?' indicates anagramming the letters (letters fly around into new positions).
'ostrich'+'i'='ostrichi'
'ostrichi' with letters rearranged gives 'HISTORIC'.
'that's' acts as a link.
(Other definitions for historic that I've seen before include "Of significance in the past" , "Pertaining to what is important in the past" , "Important and/or old" , "Connecting with old times" , "sort of present" .)