Excursionist runs in upsetting agent (7)
I believe the answer is:
tripper
'excursionist' is the definition.
(synonyms)
'runs in upsetting agent' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'upsetting agent' becomes 'tipper' (someone who tips something over).
'r' inserted into 'tipper' is 'TRIPPER'.
(Other definitions for tripper that I've seen before include "Someone out for day" , "Person on an excursion" , "Holidaymaker" , "One hoping to enjoy journey" , "One who makes an excursion" .)