Desert from the regiment when trouble starts (3)

I believe the answer is:

rat

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'desert' is the definition.
(abandon treacherously)

'regiment when trouble starts' is the wordplay.
'regiment when' becomes 'ra' (Royal Artillery. I am not sure about the 'when' bit.).
'starts' says to take the initial letters.
The initial letter of 'trouble' is 't'.
'ra'+'t'='RAT'

'from the' is the link.

Can you help me to learn more?

(Other definitions for rat that I've seen before include "Despicable person" , "One deserting sinking ship" , "Rodent - deserter" , "Rodent pest" , "Scumbag" .)

I've seen this clue in the Evening Standard.

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