Pale, greyish, with a bushy tail (4)
I believe the answer is:
ashy
'pale greyish' is the definition.
(I've seen this before)
'with a bushy tail' is the wordplay.
'with' is a charade indicator (letters next to each other).
'bushy tail' becomes 'shy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'shy' is 'ASHY'.
Can you help me to learn more?
(Other definitions for ashy that I've seen before include "Like burnt residue" , "What could tray be" , "Light grey" , "Wan" , "Deathly pale" .)