Last and final English river (6)
I believe the answer is:
endure
'last' is the definition.
(synonyms)
'final english river' is the wordplay.
'final' becomes 'end' (synonyms).
'english river' becomes 'ure' (River Ure).
'end'+'ure'='ENDURE'
'and' acts as a link.
(Other definitions for endure that I've seen before include "Suffer or undergo" , "'Last, prevail (6)'" , "Remain in existence; suffer" , "Last or suffer patiently" , "brook" .)