River Test finally divided (5)
I believe the answer is:
trent
'river' is the definition.
(I've seen this before)
'test finally divided' is the wordplay.
'finally' indicates one should take the final letters.
'divided' becomes 'rent' ('rent' can be similar in meaning to 'divided').
The last letter of 'test' is 't'.
't'+'rent'='TRENT'
(Other definitions for trent that I've seen before include "running water" , "River through Newark" , "Crossword frame" , "Notts. river" , "Main river of the Midlands" .)