Run in to check (6)
I believe the answer is:
arrest
This clue is a double definition.
'run in' is the first definition.
(thesaurus)
'check' is the second definition.
('arrest' can be a synonym of 'check')
'to' acts as a link.
(Other definitions for arrest that I've seen before include "Apprehend; detain" , "Stop or slow down the progress of" , "Stop (on suspicion)" , "Land next to the sea" , "Stop, check (progress)" .)