Impute a writer (7)
I believe the answer is:
ascribe
'impute' is the definition.
(synonyms)
'a writer' is the wordplay.
'writer' becomes 'scribe' (scribe is a kind of writer).
'a'+'scribe'='ASCRIBE'
(Other definitions for ascribe that I've seen before include "Attribute or impute" , "Attribute to, say, an old writer?" , "Attribute (something to a cause)" , "'Attribute, assign (7)'" , "Attribute to specified source or cause" .)