Policeman about to enter river, pulling up ape (7)
I believe the answer is:
copycat
'ape' is the definition.
(I've seen this before)
'policeman about to enter river pulling up' is the wordplay.
'policeman' becomes 'cop' (I've seen this before).
'about to' becomes 'ca' (I've seen this in other clues).
'enter' means one lot of letters goes inside another.
'river pulling' becomes 'ty' (I can't explain this - if you can you should give a lot more credence to this answer).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'ty' reversed gives 'yt'.
'ca' put inside 'yt' is 'ycat'.
'cop'+'ycat'='COPYCAT'
(Other definitions for copycat that I've seen before include "Plagiarist possibly" , "One imitating another" , "Slavish imitator" , "Ape" , "One who imitates another" .)