Grubby home business doubled up on ... (6)
I believe the answer is:
cocoon
'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" .)