Excessive energy used in walk (5)
I believe the answer is:
steep
'excessive' is the definition.
(thesaurus)
'energy used in walk' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'used in' means one lot of letters goes inside another.
'walk' becomes 'step' (I've seen this before).
'e' inserted within 'step' is 'STEEP'.
(Other definitions for steep that I've seen before include "pickle" , "Soak thoroughly - precipitous" , "Vertiginous; excessive" , "Almost perpendicular" , "Difficult" .)