A striking effect, on the surface (6)
I believe the answer is:
bruise
This could be a double definition.
'a striking' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'effect on the surface' is the second definition.
The definition and answer can be both to do with contact as well as being verbs in their base form.
Maybe you can see an association between them that I can't see?
Can you help me to learn more?
(Other definitions for bruise that I've seen before include "Discoloured injury" , "Discolouration left by a blow" , "Sign of injury" , "Injury mark; sounds like ferments" , "Mark left by blow" .)