A wrinkle at the hair-line (6)
I believe the answer is:
furrow
'a wrinkle' is the definition.
('furrow' can be a synonym of 'wrinkle')
'at the hair-line' is the wordplay.
'at' is a charade indicator (letters next to each other).
'the hair' becomes 'fur' (fur is a kind of hair).
'line' becomes 'row' (row is a kind of line**).
'fur' next to 'row' is 'FURROW'.
(Other definitions for furrow that I've seen before include "Groove made by plough, or wrinkle in forehead" , "'Rut, groove (6)'" , "Groove or trench" , "It's groovy!" , "Channel; wrinkle" .)