Incidentally (2,3,3)
'incidentally' is the definition.
(I know that incidentally can be written as by the way)
This is the entire clue.
(Other definitions for by the way that I've seen before include "In point of fact" , "In passing, while we're on the subject" , "At the roadside - incidentally" , "At the side of the path?" , "Incidentally, as a rather irrelevant comment" .)
'incidentally' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for by the by that I've seen before include "In passing" , "incidentally" .)