Small parts married together (2,6)
I believe the answer is:
in unison
'together' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for in unison that I've seen before include "All speaking or singing at the same time" , "All speaking or singing as one" , "All together at the same pitch" , "All singing the same notes" , "Simultaneously" .)