Slope in squiggly foreign letter (7)
I believe the answer is:
epsilon
'foreign letter' is the definition.
(letter of the Greek alphabet)
'slope in squiggly' is the wordplay.
'squiggly' indicates anagramming the letters.
'slope'+'in'='slopein'
'slopein' is an anagram of 'EPSILON'.
(Other definitions for epsilon that I've seen before include "Greek character" , "One's lip (anag)" , "Fifth brightest star in a constellation" , "Greek letter E" , "Greek short E" .)