Company opening filled by a new contract (8)
I believe the answer is:
covenant
'contract' is the definition.
(covenanting is a kind of contracting)
'company opening filled by a new' is the wordplay.
'company' becomes 'co' (abbreviation).
'opening' becomes 'vent' (vent is a kind of opening).
'filled by' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'a'+'n'='an'
'vent' going around 'an' is 'venant'.
'co'+'venant'='COVENANT'
(Other definitions for covenant that I've seen before include "Formal agreement in a convent" , "Signed agreement between two or more to do something" , "Legal agreement" , "Binding agreement between two parties" , "Formal agreement between two or more parties" .)