In agreement with firm having an event (10)
I believe the answer is:
coincident
'in agreement' is the definition.
(thesaurus)
'firm having an event' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'having' means one lot of letters go next to another.
'an event' becomes 'incident' (incident is a kind of event).
'co'+'incident'='COINCIDENT'
'with' is the link.
(Other definitions for coincident that I've seen before include "Occurring together" , "Matching" , "in the same place" .)