Ida Logan could make an oblique line (8)
I believe the answer is:
diagonal
'an oblique line' is the definition.
(I've seen this before)
'ida logan could make' is the wordplay.
'could make' indicates an anagram.
'ida'+'logan'='idalogan'
'idalogan' with letters rearranged gives 'DIAGONAL'.
(Other definitions for diagonal that I've seen before include "Line from one corner to the opposite corner" , "A straight line joining two opposite corners" , "straight line from corner" , "Slanting, from corner to corner" , "Straight line on a slant" .)