Horror of battle across river (6)
I believe the answer is:
fright
'horror' is the definition.
(both can mean fear)
'battle across river' is the wordplay.
I cannot really see how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is found in the answer.
'battle' could be 'fight' (fight is a kind of battle) and 'fight' is found in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'of' acts as a link.
(Other definitions for fright that I've seen before include "Horror" , "Grotesque" , "Panic" , "Terror" , "Start" .)