Time to remove unwanted plants in river (5)
I believe the answer is:
tweed
'river' is the definition.
(I've seen this before)
'time to remove unwanted plants' is the wordplay.
'time' becomes 't' (abbreviation).
'to remove unwanted plants' becomes 'weed' (I've seen this in another clue).
't'+'weed'='TWEED'
'in' acts as a link.
(Other definitions for tweed that I've seen before include "Thick woollen cloth - northern river" , "Running water" , "River on border between England and Scotland" , "Thick woolen fabric" , "Harris cloth" .)