Is certain to drop resistance to children (5)
I believe the answer is:
issue
'children' is the definition.
(thesaurus)
'is certain to drop resistance' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to drop' indicates named letters should be taken away.
'resistance' becomes 'r' (abbreviation).
'sure' with 'r' removed is 'sue'.
'is'+'sue'='ISSUE'
'to' is the link.
(Other definitions for issue that I've seen before include "Hot topic; outcome" , "Contentious point" , "Pour forth" , "Contentious subject" , "Point in dispute" .)