Off the plane - it's a Jumbo (8)
I believe the answer is:
elephant
'a jumbo' is the definition.
(thesaurus)
'off the plane' is the wordplay.
'off' is an anagram indicator.
'the'+'plane'='theplane'
'theplane' with letters rearranged gives 'ELEPHANT'.
'it's' acts as a link.
(Other definitions for elephant that I've seen before include "Creature" , "I'll never forget" , "Size of paper" , "Get this animal from Pat and Helen" , "bigger one from two continents?" .)