Dock policeman covering river (4)
I believe the answer is:
crop
'dock' is the definition.
(I've seen this before)
'policeman covering river' is the wordplay.
'policeman' becomes 'cop' (I've seen this before**).
'covering' means one lot of letters goes inside another.
'river' becomes 'r'.
'cop' going around 'r' is 'CROP'.
(Other definitions for crop that I've seen before include "Whip with loop instead of lash" , "Produce - short hairstyle" , "Plant grown commercially" , "It's produced at harvest time" , "Yield from a single growing season" .)