Two firms working to make a case? (6)
I believe the answer is:
cocoon
'a case?' is the definition.
(I have seen 'Pupa case ' mean 'cocoon' so perhaps 'case' could also mean 'cocoon')
'two firms working' is the wordplay.
'two firms' means two substitutions for 'firm'.
'firm' becomes 'co' (a firm is a company).
'firm' becomes 'co' (a firm is a company).
'working' becomes 'on'.
'co'+'co'+'on'='COCOON'
'to make' is the link.
(Other definitions for cocoon that I've seen before include "Swathe" , "Protect" , "secure environment" , "Wrap; pupa's protection" , "Protective case for insect" .)