A trip organised by a beast (5)
I believe the answer is:
tapir
'a beast' is the definition.
(tapir is a type of animal)
'a trip organised' is the wordplay.
'organised' indicates anagramming the letters.
'a'+'trip'='atrip'
'atrip' is an anagram of 'TAPIR'.
'by' acts as a link.
(Other definitions for tapir that I've seen before include "Long-nosed, hoofed mammal" , "creature" , "Ungulate" , "Browser" , "I part with the pig-like animal" .)