By the way, "series" is singular (7)
I believe the answer is:
strange
'singular' is the definition.
(similar in meaning)
'by the way series' is the wordplay.
'by' says to put letters next to each other.
'the way' becomes 'st' (abbreviation for street).
'series' becomes 'range' (I have seen 'Series of mountains' mean 'range' so perhaps 'series' could also mean 'range').
'st'+'range' is 'STRANGE'.
'is' is the link.
(Other definitions for strange that I've seen before include "Unusual, odd" , "Odd, peculiar" , "Bizarre" , "Surreal" , "Queer" .)