Across river, foe ultimately gets defeated (8)
I believe the answer is:
overcame
'defeated' is the definition.
(overcoming is a kind of defeating)
'across river foe ultimately' is the wordplay.
'across' becomes 'over' ('over' can be a synonym of 'across').
'river' becomes 'cam' (I've seen this before**).
'ultimately' indicates one should take the final letters.
The last letter of 'foe' is 'e'.
'over'+'cam'+'e'='OVERCAME'
'gets' is the link.
(Other definitions for overcame that I've seen before include "prevailed" , "Conquered" , "Beat (someone)" , "Defeated or got over" , "Got the better of" .)