Jumbo jet’s interior assembled in the plane (11)
I believe the answer is:
elephantine
'jumbo' is the definition.
(synonyms)
'jet's interior assembled in the plane' is the wordplay.
'interior' means to look at the middle letters.
'assembled' indicates anagramming the letters.
The centre of 'jet' is 'e'.
'in'+'the'+'plane'='intheplane'
'intheplane' is an anagram of 'lephantine'.
'e'+'lephantine'='ELEPHANTINE'
(Other definitions for elephantine that I've seen before include "like Kala Nag" , "Large and clumsy" , "Unwieldy" , "Ponderously big" , "Vast" .)