River Dart ultimately divided (5)
I believe the answer is:
trent
'river' is the definition.
(River Trent)
'dart ultimately divided' is the wordplay.
'ultimately' says to take the final letters.
'divided' becomes 'rent' (similar in meaning).
The last letter of 'dart' is 't'.
't'+'rent'='TRENT'
(Other definitions for trent that I've seen before include "Chief river of the Midlands" , "Notts. river" , "Major river of England" , "Midlands banker" , "River at Newark/Stoke" .)