Resignation more certain to involve split (9)
I believe the answer is:
surrender
'resignation' is the definition.
(I know that resignation can be written as surrender)
'more certain to involve split' is the wordplay.
'more certain' becomes 'surer' ('sure' can be a synonym of 'certain').
'to involve' means one lot of letters goes inside another (inserted letters get involved).
'split' becomes 'rend' (rend can mean to split or tear).
'surer' placed around 'rend' is 'SURRENDER'.
(Other definitions for surrender that I've seen before include "Resign" , "Cease resistance" , "Capitulation" , "Yield, give in" , "Cede" .)