Old coach’s assiduity (9)
I believe the answer is:
diligence
I believe this clue is a double definition.
'old coach's' is the first definition.
(historical type of stagecoach)
'assiduity' is the second definition.
(thesaurus)
(Other definitions for diligence that I've seen before include "Old stagecoach" , "dedication" , "Industriousness" , "Thoroughgoing application" , "Painstaking care" .)