Grey and unfashionable plaster (5)
I believe the answer is:
grout
'plaster' is the definition.
(I've seen this before)
'grey and unfashionable' is the wordplay.
'grey' becomes 'gr' (I've seen this in other clues).
'and' is a charade indicator (letters next to each other).
'unfashionable' becomes 'out' ('out' can be a synonym of 'unfashionable').
'gr'+'out'='GROUT'
(Other definitions for grout that I've seen before include "Tile fixer" , "coarsemeal" , "Fill the cracks" , "Grub" , "Thin mortar for filling cracks" .)