Length of series (5)
I believe the answer is:
chain
This clue is a double definition.
'length' is the first definition.
(I've seen this before)
'series' is the second definition.
(I know that chain is a type of series)
'of' is the link.
(Other definitions for chain that I've seen before include "Range of mountains" , "Rope of metal links" , "number of shops" , "Paper decoration" , "Length of pitch" .)