My sonny -- this needs rewriting -- word for word! (7)
I believe the answer is:
synonym
'word for word' is the definition.
(I've seen this before)
'my sonny this needs rewriting' is the wordplay.
'this needs rewriting' is an anagram indicator (I've seen this in other clues).
'my'+'sonny'='mysonny'
'mysonny' is an anagram of 'SYNONYM'.
(Other definitions for synonym that I've seen before include "Alternative word" , "A word that can be interchanged with another" , "Quick crossword clue" , "Substitute brought in for a term" , "Word that can be exchanged for another" .)