Guarantee to follow across river (6)
I believe the answer is:
ensure
'guarantee' is the definition.
('ensure' can be a synonym of 'guarantee')
'follow across river' is the wordplay.
'follow' becomes 'ensue' (I've seen this before).
'across' is an insertion indicator.
'river' becomes 'r'.
'ensue' enclosing 'r' is 'ENSURE'.
'to' is the link.
(Other definitions for ensure that I've seen before include "Make certain (something will occur)" , "Make certain, clinch" , "Protect" , "See run (anag)" , "Guarantee" .)