Worried vet heartened by British missing equestrian competition
I believe the answer is:
three-day event
'equestrian competition' is the definition.
(I've seen this before)
'worried vet heartened by british missing' is the wordplay.
'worried' indicates anagramming the letters.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'missing' is a deletion indicator.
'vet'+'heartened'+'by'='vetheartenedby'
'vetheartenedby' with 'b' removed is 'vetheartenedy'.
'vetheartenedy' anagrammed gives 'THREE-DAY EVENT'.
(Other definitions for three-day event that I've seen before include "Equestrian competition involving several skills" , "Dressage, cross-country and showjumping" , "Horse-riding competition" , "Equestrian competition comprising several disciplines" .)