Isn't straight (6)
'isn't straight' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Another definition for leans that I've seen is " Inclines, tends".)
(Other definitions for curves that I've seen before include "Bends" , "They're not straight" , "Swerves, arches" , "Arcs, bows" .)