Assailant here needs a good hiding (6)
I believe the answer is:
ambush
This could be a double definition.
'assailant here' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a good hiding' is the second definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'needs' acts as a link.
(Other definitions for ambush that I've seen before include "Wait in hiding to attack" , "Lying in wait to attack" , "Weakening" , "Attack from concealed position" , "Waylaying" .)