As one might regard it in Nine Seas (2,1,5)
I believe the answer is:
in a sense
'as' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I can't see?
'one might regard it in nine seas' is the wordplay.
'nine seas' anagrams to 'IN A SENSE'.
However, I am not clear how this is indicated.
Can you help me to learn more?
(Other definitions for in a sense that I've seen before include "After a fashion" , "According to one possible interpretation" , "To some extent" .)