Run when something wrong turns up (4)
I believe the answer is:
trot
'run' is the definition.
(I know that trot is a more specific form of the action run)
'something wrong turns up' is the wordplay.
'something wrong' becomes 'tort' (legal term for a wrongful act).
'turns up' is a reversal indicator.
'tort' written backwards gives 'TROT'.
'when' acts as a link.
(Other definitions for trot that I've seen before include "Move briskly" , "A slowish run, perhaps by horse" , "Go or walk briskly" , "Horse's steady pace" , "Fairly fast pace of a horse" .)