Clean egg is in return packaging (8)
I believe the answer is:
sanitise
'clean' is the definition.
('clean'->'sanitize' is in my internal thesaurus and 'ize'->'ise')
'egg is in return packaging' is the wordplay.
'egg' becomes 'nit' (a nit is an insect egg).
'in' means one lot of letters goes inside another.
'return packaging' becomes 'sae' (I am not sure about this - if you are sure you should believe this answer much more).
'nit'+'is'='nitis'
'nitis' inserted into 'sae' is 'SANITISE'.
Can you help me to learn more?
(Other definitions for sanitise that I've seen before include "Clean up, make hygienic like a Sistine" , "Clean and sterilize" , "Make hygienic being in siesta" , "Make hygienic or more acceptable" , "disinfect" .)