Thread; sequence (6)
I believe the answer is:
string
I believe this clue is a double definition.
'thread' is the first definition.
(I know that thread can be written as string)
'sequence' is the second definition.
(I know that string is a type of sequence)
(Other definitions for string that I've seen before include "It's difficult to say how long a piece of it is" , "How long could a piece of it be?" , "Something for tying up" , "Hard to say how long a piece of it is" , "groups of horses" .)