Those cocooning with the young boxer Russell (5)
I believe the answer is:
pupae
'those cocooning' is the definition.
I can't tell whether this defines the answer.
'with the young boxer russell' is the wordplay.
'with' means one lot of letters go next to another.
'the young boxer' becomes 'pup' (boxer is a type of dog).
'russell' becomes 'ae' (I can't justify this - if you can you should give a lot more credence to this answer).
'pup'+'ae' is 'PUPAE'.
Can you help me to learn more?
(Other definitions for pupae that I've seen before include "Insects between larval and adult stages" , "Insects in the inactive stage of development" , "Insects in form between larvae and adults" , "Chrysalids" , "Chrysalises" .)