Start defending one viewpoint at length (8)
I believe the answer is:
distance
'length' is the definition.
(I know that length can be written as distance)
'start defending one viewpoint' is the wordplay.
'start' indicates taking the first letters.
'one' becomes 'I' (Roman numeral).
'viewpoint' becomes 'stance' (I've seen this before).
The initial letter of 'defending' is 'd'.
'd'+'i'+'stance'='DISTANCE'
'at' acts as a link.
(Other definitions for distance that I've seen before include "Length of space between one place and another" , "Range" , "A far-off point" , "to make big winning margin" , "Space between things" .)