Resignation is more certain to involve split (9)
I believe the answer is:
surrender
'resignation' is the definition.
(surrender is a kind of resignation)
'more certain to involve split' is the wordplay.
'more certain' becomes 'surer' ('sure' can be similar in meaning to 'certain').
'to involve' is an insertion indicator.
'split' becomes 'rend' (I've seen this in another clue).
'surer' going around 'rend' is 'SURRENDER'.
'is' is the link.
(Other definitions for surrender that I've seen before include "submission" , "Capitulation" , "Cease resistance" , "Submit (to an opponent)" , "Cede" .)