Certain about one insect’s 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 about one insect's' is the wordplay.
'certain' becomes 'sure' (synonyms).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'insect' becomes 'gnat' (gnat is a kind of insect).
'i'+'gnat'='ignat'
'sure' going around 'ignat' is 'SIGNATURE'.
'identifying' acts as a link.
(Other definitions for signature that I've seen before include "Autograph" , "time indicated by this?" , "ending to letter?" , "proof of identity" , "Identifying piece of writing, tune etc" .)