Rage at answer to this clue, then have penny-drop moment (3,3,5)
I believe the answer is:
see the light
'have penny-drop moment' is the definition.
(both can mean to understand something suddenly)
'rage at answer to this clue' is the wordplay.
'rage' becomes 'seethe' (I've seen this in another clue).
'at' says to put letters next to each other.
'answer to this clue' becomes 'light' (spaces for answers in crosswords are called lights).
'seethe'+'light'='SEE THE LIGHT'
'then' acts as a link.
(Other definitions for see the light that I've seen before include "Become converted" , "get it?" , "Realise the error of one's ways" , "Gain sudden insight - be religiously converted" , "Be born" .)