Head cut from nail with two straight lines (5)
I believe the answer is:
radii
'straight lines' is the definition.
(radius is a kind of straight line)
'head cut from nail with two' is the wordplay.
I cannot quite understand how this works, but
'two' could be 'ii' (Roman numerals) and 'ii' is located in the answer.
The remaining letters 'rad' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for radii that I've seen before include "Forearm bones: spokes" , "Straight lines from centre of a circle" , "Lines between the center and circumference of a circle" , "spoke with others" , "Lines from centre to perimeter of a circle" .)