Almost certain to break up - that's appropriate (5)
I believe the answer is:
usurp
'appropriate' is the definition.
(thesaurus)
'almost certain to break up' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to break' is an insertion indicator.
'sure' with its last letter taken away is 'sur'.
'sur' going within 'up' is 'USURP'.
'that's' acts as a link.
(Other definitions for usurp that I've seen before include "Seize (throne, e.g.)" , "Seize without right" , "Appropriate without authority" , "Take power unlawfully" , "Seize power without authority" .)