Awesome, two straight lines (5)
I believe the answer is:
radii
'straight lines' is the definition.
(radius is a kind of straight line)
'awesome two' is the wordplay.
'awesome' becomes 'rad' (I've seen this in another clue).
'two' becomes 'II' (Roman numerals).
'rad'+'ii'='RADII'
(Other definitions for radii that I've seen before include "Lines between the center and circumference of a circle" , "Straight lines from the centre to the surface of a sphere" , "lines from a central part" , "spoke with others" , "Lines from centre to perimeter of a circle" .)