The plane, possibly, is a Jumbo! (8)
I believe the answer is:
elephant
'a jumbo' is the definition.
(Jumbo was an elephant who lived in London Zoo)
'the plane possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'the'+'plane'='theplane'
'theplane' is an anagram of 'ELEPHANT'.
'is' acts as a link.
(Other definitions for elephant that I've seen before include "Trumpeting animal" , "Largest living land animal" , "The plane (anag.)" , "Jumbo" , "The animal that never forgets" .)