Ultimately unpleasant eccentric in Sweeney? (4)
I believe the answer is:
todd
'sweeney?' is the definition.
(Sweeney Todd)
'ultimately unpleasant eccentric' is the wordplay.
'ultimately' says to take the final letters.
'eccentric' becomes 'odd' (odd can mean eccentric or unusual).
The final letter of 'unpleasant' is 't'.
't'+'odd'='TODD'
'in' is the link.
(Other definitions for todd that I've seen before include "Demon 12 of Fleet Street" , "19th-century demon barber of Fleet Street" , "Irish-born British actor and soldier, Richard, d. 2009" , "Richard --, was Guy Gibson in The Dam Busters" , "Sweeney, Fleet Street barber" .)