To run when trouble starts is a bad thing (4)
I believe the answer is:
'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'.
'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" .)