Sidekick reported theft? (5)

I believe the answer is:
robin
This clue could be a double definition.
'sidekick' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'reported theft?' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
Can you help me to learn more?
(Other definitions for robin that I've seen before include "good fellow in the D [DREAM]" , "See 12" , "Christmas-card bird" , "Small brown and red bird" , "Bird - one in Lincoln green?" .)
