Two companies joining together on a silk product (6)
I believe the answer is:
cocoon
'a silk product' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'two companies joining together' is the wordplay.
'two companies' means two substitutions for 'company'.
'company' becomes 'co' (abbreviation).
'company' becomes 'co' (abbreviation).
'joining together' becomes 'on' (I am not sure about this - if you are sure you should believe this answer much more).
'co'+'co'+'on'='COCOON'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for cocoon that I've seen before include "Swathe" , "Silken sheath" , "Protection for pupa" , "Protective cover of silkworms, etc" , "Pupa's protective case" .)