It only occurs to one later (12)

I believe the answer is:
afterthought
(Other definitions for afterthought that I've seen before include "Item added later" , "not an original idea" , "Later reassessment" , "PS for instance" , "A reflection" .)
I believe the answer is:
afterthought
(Other definitions for afterthought that I've seen before include "Item added later" , "not an original idea" , "Later reassessment" , "PS for instance" , "A reflection" .)