Cross, having endless pain in face (8)
I believe the answer is:
diagonal
'cross' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'endless pain in face' is the wordplay.
'endless pain' becomes 'agon' (I've seen this in other clues).
'in' indicates putting letters inside.
'face' becomes 'dial' (face of a clock).
'agon' put within 'dial' is 'DIAGONAL'.
'having' acts as a link.
Can you help me to learn more?
(Other definitions for diagonal that I've seen before include "On a slope" , "Straight line on a slant" , "bishop moves here" , "Line from one corner to the fartherst one" , "It joins square's opposite corners" .)