Unions strangely in agreement (6)
I believe the answer is:
unison
'in agreement' is the definition.
(I've seen this before)
'unions strangely' is the wordplay.
'strangely' is an anagram indicator.
'unions' with letters rearranged gives 'UNISON'.
(Other definitions for unison that I've seen before include "concert" , "Accord" , "As one voice" , "with regard to the pitch" , "Complete agreement, harmony" .)