Drop in portentous energy in contest (8)
I believe the answer is:
disagree
'contest' is the definition.
(thesaurus)
'drop in portentous energy' is the wordplay.
'drop' becomes 'sag' (sagging is a kind of dropping).
'in' means one lot of letters goes inside another.
'portentous' becomes 'dire' (I've seen this before).
'energy' becomes 'e' (physics symbol as in E=mca).
'sag' put within 'dire' is 'disagre'.
'disagre'+'e'='DISAGREE'
'in' is the link.
(Other definitions for disagree that I've seen before include "Row" , "Differ in opinion" , "Take issue with, hold different opinion" , "Hold a differing opinion" , "Differ, take issue" .)