Police operation in region hard to understand on reflection (5,4)
I believe the answer is:
speed trap
'police operation' is the definition.
(I've seen this before)
'in region hard to understand on reflection' is the wordplay.
'in' means one lot of letters goes inside another.
'region' becomes 'parts' (parts is a kind of region).
'hard to understand' becomes 'deep' (deep can mean profound or difficult to understand).
'on reflection' says the letters should be written in reverse.
'parts' enclosing 'deep' is 'partdeeps'.
'partdeeps' backwards is 'SPEED TRAP'.
(Other definitions for speed trap that I've seen before include "Set-up to catch fast drivers" , "Police control" , "inconvenience for boy racers?" , "that slows motorist?" , "something drivers might avoid" .)