A connected series or group (5)
I believe the answer is:
nexus
I believe this is a double definition.
'a connected series' is the first definition.
(I've seen this in another clue)
'group' is the second definition.
(nexus is a kind of group)
(Other definitions for nexus that I've seen before include "Connecting bond" , "Bond; linked group" , "Focal point; link" , "line" , "joined group" .)