Three climbing egg on a communist riled up (9)
I believe the answer is:
triggered
'riled up' is the definition.
The definition and answer can be both to do with contact as well as being past participle verbs.
Maybe you can see a link between them that I can't see?
'three climbing egg on a communist' is the wordplay.
'three' becomes 'tri' (prefix meaning 'three').
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'on' means one lot of letters go next to another.
'a communist' becomes 'red' (red can informally mean communist).
'egg' backwards is 'gge'.
'tri'+'gge'+'red'='TRIGGERED'
Can you help me to learn more?
(Other definitions for triggered that I've seen before include "Caused (something) to happen" , "Set off" , "were let off" , "gets fired" .)