Company opening involving an agreement (8)
I believe the answer is:
covenant
'agreement' is the definition.
(I know that covenant is a type of agreement)
'company opening involving an' is the wordplay.
'company' becomes 'co' (abbreviation).
'opening' becomes 'vent' (vent is a kind of opening).
'involving' means one lot of letters goes inside another.
'vent' placed around 'an' is 'venant'.
'co'+'venant'='COVENANT'
(Other definitions for covenant that I've seen before include "Signed agreement between two or more to do something" , "Formal agreement in a convent" , "Binding contract" , "Formal binding agreement" , "Promise" .)