By the sound of it, go lower in harmony (4)
I believe the answer is:
sync
'in harmony' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for sync that I've seen before include "go together" , "Relationship when things occur at the same time" , "Togetherness (of sound and picture, say)" , "In which things occur simultaneously" , "In which one's matching" .)