To run when trouble starts is a bad thing (4)

I believe the answer is:

trot

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'to run' is the definition.
(trotting is a kind of running)

'trouble starts is a bad thing' is the wordplay.
'starts' suggests taking the first letters.
'is a bad thing' becomes 'rot' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'trouble' is 't'.
't'+'rot'='TROT'

'when' is the link.

Can you help me to learn more?

(Other definitions for trot that I've seen before include "Brisk movement" , "Red" , "Militant socialist (informal)" , "Radical socialist (abbr) - horse gait" , "Move at brisk pace" .)

I've seen this clue in the Evening Standard.

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