Informally, handles collars worn by celebrity (9)
I believe the answer is:
nicknames
'informally handles' is the definition.
'nicknames' can be an answer for 'handles' (I've seen this before). I'm not certain of the 'informally' bit.
'collars worn by celebrity' is the wordplay.
'collars' becomes 'nicks' (I can't justify this - if you can you should give a lot more credence to this answer).
'worn by' indicates putting letters inside (I've seen this in other clues).
'celebrity' becomes 'name' (I've seen this before).
'nicks' placed around 'name' is 'NICKNAMES'.
Can you help me to learn more?
(Other definitions for nicknames that I've seen before include "Chalky and Shorty, for instance" , "Monikers" , "Handles" , "Familiar or affectionate add-on appellations" , "sobriquets" .)