Rough lines outlining snake (5)
I believe the answer is:
'rough' is the definition.
(I know that rough can be written as raspy)
'lines outlining snake' is the wordplay.
'lines' becomes 'ry' (abbreviation for railway).
'outlining' means one lot of letters goes inside another (some letters create an outline round others).
'snake' becomes 'asp' (asp is a kind of snake).
'ry' enclosing 'asp' is 'RASPY'.
(Another definition for raspy that I've seen is " Harsh, grating".)