Obligation of French telecom company (4)
I believe the answer is:
debt
'obligation' is the definition.
(I know that debt is a type of financial obligation)
'of french telecom company' is the wordplay.
'of french' becomes 'de' ('of' in French).
'telecom company' becomes 'BT' (British Telecom).
'de'+'bt'='DEBT'
(Other definitions for debt that I've seen before include "Amount owed" , "Commitment" , "Amount owing" , "Arrears, liability" , "Sum of money that's owed" .)