Sudden panic made an impression on egghead (8)
I believe the answer is:
stampede
'sudden panic' is the definition.
'stampede' can be an answer for 'panic' (thesaurus). I'm not certain of the 'sudden' bit.
'made an impression on egghead' is the wordplay.
'made an impression' becomes 'stamped' (I've seen this before).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'egghead' becomes 'e' (head letter of 'egg').
'stamped'+'e'='STAMPEDE'
Can you help me to learn more?
(Other definitions for stampede that I've seen before include "Sudden mad rush" , "Run in panic" , "Headlong rush of animals" , "Sudden dash of a group of startled animals" , "collective flight" .)