Forgive and forget (3,2,2)
I think it could be either:
let bygones be bygones
'forgive and forget' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for let it go that I've seen before include "Hit song from Disney's 2013 animated feature film Frozen" , "Refrain from comment" , "Excuse something" , "Disney tune" .)
(Other definitions for let bygones be bygones that I've seen before include "not being able to revalue old artefacts" , "Be reconciled" .)