Tire, having to employ energy in part of plane (8)
I believe the answer is:
fuselage
'part of plane' is the definition.
(I know that fuselage is part of airplane)
'tire having to employ energy' is the wordplay.
'tire' becomes 'flag' (I've seen this in another clue).
'having' indicates putting letters inside.
'to employ' becomes 'use' (synonyms).
'energy' becomes 'e' (physics symbol as in E=mc2).
'flag' enclosing 'use' is 'fuselag'.
'fuselag'+'e'='FUSELAGE'
'in' is the link.
(Other definitions for fuselage that I've seen before include "Long body" , "Body of plane" , "Main body of aircraft" , "bodywork" , "part of aircraft" .)