Lose it completely, chucking policeman in river (7)
I believe the answer is:
explode
'lose it' is the definition.
(I've seen this in another clue)
'policeman in river' is the wordplay.
'policeman' becomes 'plod' (UK informal term for a police officer).
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'plod' put into 'exe' is 'EXPLODE'.
'completely chucking' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for explode that I've seen before include "Blow up" , "mushroom" , "Go off with a bang" , "Pop" , "Burst outward" .)