Animal one caught in dreadful trap (5)
I believe the answer is:
tapir
'animal' is the definition.
(tapir is a type of animal)
'one caught in dreadful trap' is the wordplay.
'one' becomes 'i' (Roman numeral).
'caught in' means one lot of letters goes inside another.
'dreadful' is an anagram indicator.
'trap' anagrammed gives 'tapr'.
'i' inserted inside 'tapr' is 'TAPIR'.
(Other definitions for tapir that I've seen before include "I part with the pig-like animal" , "Hoofed mammal; I trap (anag.)" , "Long-nosed, hoofed mammal" , "Tropical hoofed mammal with a flexible proboscis" , "Browser" .)