Crossword Genius

What sort of jerk would keep lawman on the case? (4)

Ross

I believe the answer is:

soda

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

'what sort of jerk would keep' is the definition.
I can't tell whether this definition defines the answer.

'lawman on the case?' is the wordplay.
'lawman' becomes 'da'.
'on' is a charade indicator (letters next to each other).
'the case?' becomes 'so' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'da' after 'so' is 'SODA'.

Can you help me to learn more?

(Other definitions for soda that I've seen before include "Soft drink" , "Fizzy drink - chemical compound" , ""Splashed" drinks mixer" , "Accompaniment to whisky" , "Splash served with some spirits" .)

I've seen this clue in The Times.
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