Certain to include bizarre giant identifying mark (9)
I believe the answer is:
signature
'mark' is the definition.
(I have seen 'Distinctive mark ' mean 'signature' so perhaps 'mark' could also mean 'signature')
'certain to include bizarre giant' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to include' is an insertion indicator.
'bizarre' is an anagram indicator.
'giant' anagrammed gives 'ignat'.
'sure' placed around 'ignat' is 'SIGNATURE'.
'identifying' acts as a link.
(Other definitions for signature that I've seen before include "Identifying script" , "(In music) the sharps or flats that follow the clef and indicate the key" , "Autograph" , "Distinctive way of writing name" , "One's mark" .)