Absorb black a great deal (4)
I believe the answer is:
blot
'absorb' is the definition.
(blotting is a kind of absorbing)
'black a great deal' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'a great deal' becomes 'lot' (both can mean a large amount).
'b'+'lot'='BLOT'
(Other definitions for blot that I've seen before include "Soak up (ink); blemish" , "Spot of ink, paint etc" , "Dry piece of writing" , "Don't do it to your copybook" , "Ink stain; disgrace" .)