Say, an act with nothing even in harmony (4)
I believe the answer is:
sync
'harmony' is the definition.
(I've seen this in another clue)
'say an act with nothing even in' is the wordplay.
'with nothing even in' indicates alternate letters (letters in even-numbered positions removed).
'say'+'an'+'act'='sayanact'
The alternating letters of 'sayanact' are 'SYNC'.
(Other definitions for sync that I've seen before include "go together" , "Togetherness (of sound and picture, say)" , "In which things occur simultaneously" , "Relationship when things occur at the same time" , "Unison (abbrev.)" .)