long-standing
adjective /ˌlɒŋ ˈstændɪŋ/
  /ˌlɔːŋ ˈstændɪŋ/
[usually before noun]- that has existed or lasted for a long time
- a long-standing relationship
 - Long-standing grievances were aired at the meeting.
 - The country’s long-standing relationship with the US was finally under strain.
 
Oxford Collocations DictionaryLong-standing is used with these nouns:- advocate
 - ambition
 - animosity
 - …