Line of roentgen found in unstable nickel (7)
I believe the answer is:
crinkle
'line' is the definition.
('crinkle' can be a synonym of 'line')
'roentgen found in unstable nickel' is the wordplay.
'roentgen' becomes 'r' ('R' can be a synonym of 'roentgen').
'found in' means one lot of letters goes inside another.
'unstable' indicates an anagram.
'nickel' is an anagram of 'cinkle'.
'r' put inside 'cinkle' is 'CRINKLE'.
'of' is the link.
(Other definitions for crinkle that I've seen before include "Clinker (anag.)" , "Curl" , "Small crease or wrinkle" .)