Having pale greyish colour when extremely hoary (4)
I believe the answer is:
ashy
'having pale greyish colour' is the definition.
(the colour of ash)
'when extremely hoary' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'extremely' suggests removing the centre (only the extremes of the word are used).
'hoary' with its centre taken out is 'hy'.
'as'+'hy'='ASHY'
(Other definitions for ashy that I've seen before include "Leaden" , "evidence of recent fire" , "Light grey" , "Like burnt wood or coal" , "covered in dust" .)