A box? Rot! Has to be packed in a tin (6)
I believe the answer is:
carton
'a tin' is the definition.
(thesaurus)
'a box? rot has to be packed in' is the wordplay.
'box?' indicates an anagram (I've seen 'box' mean this (the letters are knocked about)).
'has' means one lot of letters go next to another.
'to be packed' becomes 'cn' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'rot' is an anagram of 'rto'.
'a'+'rto'='arto'
'arto' inserted inside 'cn' is 'CARTON'.
Can you help me to learn more?
(Other definitions for carton that I've seen before include "Container for food or liquid" , "Box without great strength" , "Small, light box or container" , "Flapped cardboard box" , "Box made of cardboard" .)