Engineers checking old-fashioned circuit (5)
I believe the answer is:
route
'circuit' is the definition.
(thesaurus)
'engineers checking old-fashioned' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'checking' means one lot of letters goes inside another (check can mean to control or arrest).
'old-fashioned' becomes 'out' (as opposed to 'in').
're' enclosing 'out' is 'ROUTE'.
(Other definitions for route that I've seen before include "Course to be travelled" , "Send a particular way" , "The way to get somewhere" , "Course of 7" , "Choice of roads in an itinerary" .)