Finally get it (3,3,5)
I believe the answer is:
see the light
'finally get it' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for see the light that I've seen before include "Become converted" , "Gain sudden insight - be religiously converted" , "Realise one's mistake" , "Be born" , "get it?" .)