Writer of letter US prosecutor dismissed (4)
I believe the answer is:
lamb
'writer' is the definition.
(I've seen this before)
'letter us prosecutor dismissed' is the wordplay.
'letter' becomes 'lambda' (letter of the Greek alphabet).
'us prosecutor' becomes 'DA' (district attorney).
'dismissed' suggests deleting specific letters.
'lambda' with 'da' taken away is 'LAMB'.
'of' is the link.
(Other definitions for lamb that I've seen before include "Noted essayist - symbol of innocence" , "Tender sheep meat" , "Young farm animal" , "Skipper" , "Meaty" .)