Lower River Test cutting (6)
I believe the answer is:
demote
'lower' is the definition.
(I've seen this in another clue)
'river test cutting' is the wordplay.
'river' becomes 'dee' (River Dee).
'test' becomes 'MOT' (as in an MOT test for a vehicle).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'dee' going around 'mot' is 'DEMOTE'.
(Other definitions for demote that I've seen before include "Relegate" , "Move down in rank" , "Reduce in status" , "Reduce to lower rank as punishment" , "Downgrade" .)