Start of a reminiscence (4)
I believe the answer is:
once
'start of a reminiscence' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for once that I've seen before include "One-time musical film and song" , "Long ago" , "not repeatedly" , "On a single (or past) occasion" , "in bygone days" .)