The colour of ash (4)
I believe the answer is:
grey
'the colour of ash' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grey that I've seen before include "Colour, between black and white" , "Unclear (area); dull (weather)" , "Bushy-tailed North America rodent, now a UK pest" , "11 - if of 10, white" , "The colour of ash" .)