Excessive energy used in part of flight (5)
I believe the answer is:
steep
'excessive' is the definition.
(thesaurus)
'energy used in part of flight' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mct).
'used in' is an insertion indicator.
'part of flight' becomes 'step' (I can't explain this - if you can you should believe this answer much more).
'e' put inside 'step' is 'STEEP'.
Can you help me to learn more?
(Other definitions for steep that I've seen before include "over the top" , "pickle" , "Precipitous - exorbitant" , "Rapidly rising" , "not reasonable" .)