Protect two companies, one briefly (6)
I believe the answer is:
cocoon
'protect' is the definition.
(I've seen this before)
'two companies one briefly' is the wordplay.
'two companies' means two replacements for 'company'.
'company' becomes 'co' (abbreviation).
'company' becomes 'co' (abbreviation).
'briefly' means to remove the last letter.
'one' with its final letter taken off is 'on'.
'co'+'co'+'on'='COCOON'
(Other definitions for cocoon that I've seen before include "Pupa's protective case" , "Envelop for protection" , "Larva case" , "Silky case protecting a pupa" , "Protective covering around some animal eggs" .)