Aggression displayed by curt, uppity, oddly ugly eunuch with energy (10)
I believe the answer is:
truculence
'aggression' is the definition.
(I've seen this before)
'curt uppity oddly ugly eunuch with energy' is the wordplay.
'uppity' says the letters should be written backwards.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'ugly'+'eunuch'='uglyeunuch'
The alternating letters of 'uglyeunuch' are 'ulenc'.
'curt' back-to-front is 'truc'.
'truc'+'ulenc'+'e'='TRUCULENCE'
'displayed by' acts as a link.
(Other definitions for truculence that I've seen before include "Aggression" , "Bellicosity" , "Aggressive defiance" .)