Speed up attack (4)
I believe the answer is:
rush
I believe this is a double definition.
'speed up' is the first definition.
(thesaurus)
'attack' is the second definition.
(rush can mean to attack with great force)
(Other definitions for rush that I've seen before include "Speed along - waterside plant" , "Dash to the grassy plant" , "Be too hasty or too fast" , "High" , "Liverpool footballer once" .)