A sloping letter (6)
I believe the answer is:
italic
'a sloping letter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for italic that I've seen before include "more oblique than Roman" , "Printing style used for emphasis" , "not exactly an upright type" , "Principally it" , "Relating to letters sloping to the right" .)