One identifying secret agent's features (3)
I believe the answer is:
tag
'one identifying' is the definition.
(a tag or label identifies something)
'secret agent's features' is the wordplay.
'features' indicates a hidden word (I've seen 'featuring' mean this).
'TAG' is hidden within 'secret agents'.
(Other definitions for tag that I've seen before include "Children's chasing game" , "Tie-on label" , "dog" , "day in Berlin" , "Name" .)