Crossword Genius

Roles profound back in motorists' hazard (5,4)

Ross

I believe the answer is:

speed trap

I'm a little stuck... Click here to teach me more about this clue!

'motorists' hazard' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'roles profound back in' is the wordplay.
'roles' becomes 'strap' (I can't justify this - if you can you should give a lot more credence to this answer).
'profound' becomes 'deep' (synonyms).
'back' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'deep' reversed gives 'peed'.
'strap' placed around 'peed' is 'SPEED TRAP'.

Can you help me to learn more?

(Other definitions for speed trap that I've seen before include "inconvenience for boy racers?" , "police operation" , "Police control" , "Hazard for fast motorist" , "something drivers might avoid" .)

I've seen this clue in the Sydney Morning Herald.
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