Crime writer succeeded when introducing the ultimate character in Ellery Queen (6)
I believe the answer is:
sayers
'crime writer' is the definition.
(Dorothy L Sayers)
'succeeded when introducing the ultimate character in ellery queen' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'when' becomes 'as'.
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'the ultimate character in' indicates one should take the final letters.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
The last letter of 'ellery' is 'y'.
'y'+'er'='yer'
'as' placed around 'yer' is 'ayers'.
's'+'ayers'='SAYERS'
(Other definitions for sayers that I've seen before include "Detective fiction author, d. 1957" , "Dorothy L. ..., British crime writer" , "detective story writer" , "they announce" , "Dorothy L. --, novelist" .)