Gave the impression, when one happened on (4,6)
I believe the answer is:
came across
'gave the impression' is the definition.
The answer and definition can be both related to thinking as well as being past participle verbs.
Perhaps there's a link between them I don't understand?
'one happened on' is the wordplay.
'one' becomes 'across' (I can't justify this - if you can you should believe this answer much more).
'happened' becomes 'came' (coming is a kind of happening).
'on' says to put letters next to each other.
'across' put after 'came' is 'CAME ACROSS'.
'when' acts as a link.
Can you help me to learn more?
(Another definition for came across that I've seen is " Found".)