A wrinkle by the hair-line (6)
I believe the answer is:
furrow
'a wrinkle' is the definition.
(I know that wrinkle can be written as furrow)
'by the hair-line' is the wordplay.
'by' is a charade indicator (letters next to each other).
'the hair' becomes 'fur' (fur is a kind of hair).
'line' becomes 'row' (a row or line of things).
'fur' next to 'row' is 'FURROW'.
(Other definitions for furrow that I've seen before include "'Rut, groove (6)'" , "Plough's rut" , "Ploughing mark" , "Channel; wrinkle" , "A long groove or deep wrinkle" .)