As far as it goes (6)
I believe the answer is:
length
'as' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for length that I've seen before include "A bit of swimming" , "Linear extent" , "Something's largest dimension" , "A brief swim" , "stretch" .)