Crossword Genius

Grubby home business doubled up on ... (6)

Ross

I believe the answer is:

cocoon

I'm a little stuck... Click here to teach me more about this clue!

'grubby home' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'business doubled up on' is the wordplay.
'business' becomes 'co' (short for company).
'doubled up' becomes 'co' (I can't justify this - if you can you should give a lot more credence to this answer).
'co'+'co'+'on'='COCOON'

Can you help me to learn more?

(Other definitions for cocoon that I've seen before include "secure environment" , "Protective case for insect" , "Protective covering around some animal eggs" , "Silken sheath" , "Silky sheath around a chrysalis" .)

I've seen this clue in the Sydney Morning Herald.
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