Cannot fail to include child in agreement (9)
I believe the answer is:
consonant
'in agreement' is the definition.
(synonyms)
'cannot fail to include child' is the wordplay.
'fail' indicates anagramming the letters (I've seen 'failed' mean this).
'to include' indicates putting letters inside.
'child' becomes 'son' (son is a kind of child).
'cannot' with letters rearranged gives 'conant'.
'conant' enclosing 'son' is 'CONSONANT'.
(Other definitions for consonant that I've seen before include "One of several characters" , "Only one of Ouida's characters is" , "not I, nor you, one hears" , "In agreement" , "Speech sound, not a vowel" .)