in a sense; in some way; to an extent
Used to qualify a statement, indicating that it is true from a certain perspective or to a limited degree. Often softens assertions.
In a sense, what he says is correct.
This movie is scary in some way.
ある意味で qualifies a claim as true from one angle, while 一種 softens a claim by calling something 'a kind of' the thing described.
ある意味では and ある意味で are interchangeable, but では is more common in speech while で can feel a touch more formal or written.
Phrase composed of ある (a certain) + 意味 (meaning) + で (particle indicating means or manner). Literally 'in a certain meaning'.