Knowledge accepted without thanks (3)
I believe the answer is:
ken
'knowledge' is the definition.
(ken is a kind of knowledge)
'accepted without thanks' is the wordplay.
'accepted' becomes 'taken' ('take' can be a synonym of 'accept').
'without' suggests deleting specific letters.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'taken' with 'ta' removed is 'KEN'.
(Other definitions for ken that I've seen before include "Awareness" , "One's range of knowledge" , "Notice" , "Identify; -- Livingstone" , "Know; man's name" .)