Certain result to invest Republican — and Democrat! (7)
I believe the answer is:
ensured
'certain' is the definition.
(I've seen this before)
'result to invest republican and democrat' is the wordplay.
'result' becomes 'ensue' (to ensue is to occur or result).
'to invest' indicates putting letters inside (back).
'republican' becomes 'r' (abbreviation).
'and' says to put letters next to each other.
'democrat' becomes 'd'.
'ensue' enclosing 'r' is 'ensure'.
'ensure'+'d'='ENSURED'
(Other definitions for ensured that I've seen before include "Saw to it" , "Made certain, clinched" , "Determined" , "guaranteed" , "Made certain for end user" .)