Attackers battle to block river crossings (8)
I believe the answer is:
forwards
'attackers' is the definition.
(I've seen this before)
'battle to block river crossings' is the wordplay.
'battle' becomes 'war' (I've seen this before).
'to block' means one lot of letters goes inside another (block can mean to fill a hole in something).
'river crossings' becomes 'fords' (I've seen this before).
'war' going inside 'fords' is 'FORWARDS'.
(Other definitions for forwards that I've seen before include "strikers" , "pack" , "Attacking players" , "Football players" , "In the normal sequence" .)