Greek letter varies in slope (7)
I believe the answer is:
epsilon
'greek letter' is the definition.
(letter from the Greek alphabet)
'varies in slope' is the wordplay.
'varies' indicates anagramming the letters (I've seen 'varying' mean this).
'in'+'slope'='inslope'
'inslope' with letters rearranged gives 'EPSILON'.
(Other definitions for epsilon that I've seen before include "Greek letter - pinoles (anag)" , "foreign letter" , "Greek letter E" , "Lose pin (anag.)" , "Fifth brightest star in a constellation" .)