The agreement is, one takes the child (6)
I believe the answer is:
unison
'the agreement' is the definition.
(unison is a kind of agreement)
'one takes the child' is the wordplay.
'one' becomes 'unit' (synonyms).
'takes' is a deletion indicator.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'child' becomes 'son' (male child).
'unit' with 't' taken out is 'uni'.
'uni'+'son'='UNISON'
'is' acts as a link.
(Other definitions for unison that I've seen before include "Agreement or concord, as with choir" , "all players directed to the same pitch" , "Unscrupulous dealer" , "All singing the same note" , "Complete agreement, harmony" .)