In the plane, converted energy is huge (11)
I believe the answer is:
elephantine
'huge' is the definition.
(I've seen this before)
'in the plane converted energy' is the wordplay.
'converted' indicates an anagram.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in'+'the'+'plane'='intheplane'
'intheplane' is an anagram of 'elephantin'.
'elephantin'+'e'='ELEPHANTINE'
'is' acts as a link.
(Other definitions for elephantine that I've seen before include "Ponderously big" , "Huge" , "like Kala Nag" , "Heavily big" , "Vast" .)