A tendency to lose a set course (5)
I believe the answer is:
track
I believe this is a quadruple definition!!
'a tendency' is the first definition.
(thesaurus)
'to lose' is the second definition.
(thesaurus)
'a set' is the third definition.
(thesaurus)
'course' is the fourth definition.
(synonyms)
(Other definitions for track that I've seen before include "Extract from record" , "Shadow" , "path to follow" , "Parallel rails for wheels" , "one of a number on record" .)