Impute (7)
I believe the answer is:
'impute' is the definition.
(I know that impute is a more specific form of ascribe)
This is all the clue.
(Other definitions for ascribe that I've seen before include "Attribute to, say, an old writer?" , "'Impute, assign (7)'" , "Attribute (something to a cause)" , "Attribute or impute" , "Attribute to specified source or cause" .)