Noticing flying object, initially unidentified (9)
I believe the answer is:
incognito
'unidentified' is the definition.
(thesaurus)
'noticing flying object initially' is the wordplay.
'flying' is an anagram indicator (letters fly around into new positions).
'initially' suggests taking the first letters.
The initial letter of 'object' is 'o'.
'noticing' anagrammed gives 'incognit'.
'incognit'+'o'='INCOGNITO'
(Other definitions for incognito that I've seen before include "Under an assumed name" , "Covertly" , "under another name" , "With true identity concealed" , "as disguised person appears?" .)