Over the length of (5)
I believe the answer is:
along
'over the length of' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for along that I've seen before include "With a forward motion: in accompaniment" , "Lengthwise" , "Sea monster, opposite Charybdis" , "From one end to the other" , "Moving in a constant direction on (something)" .)