Third-level places working in harmony (6)
I believe the answer is:
unison
'in harmony' is the definition.
(thesaurus)
'third-level places working' is the wordplay.
'third level places' becomes 'unis' (I can't justify this - if you can you should give a lot more credence to this answer).
'working' becomes 'on' (on can mean working or activated).
'unis'+'on'='UNISON'
Can you help me to learn more?
(Other definitions for unison that I've seen before include "concert" , "with regard to the pitch" , "The concordant trade union?" , "Type of singing" , "Agreement or concord, as with choir" .)