Absorbs fluid, including ink (6)
I believe the answer is:
drinks
'absorbs' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how one could define the other.
'fluid including ink' is the wordplay.
'fluid' becomes 'drink' (drink is a kind of fluid).
'including ink' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'drink'+'s'='DRINKS'
Can you help me to learn more?
(Other definitions for drinks that I've seen before include "they are to be swallowed" , "Quaffs, imbibes" , "Swallows liquids" , "Consumes alcohol" , "Beverages" .)