Temporary substitute (7)
'temporary substitute' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for stopgap that I've seen before include "Temporary measure" , "Temporary fix" , "Makeshift" , "Improvisation" , "caretaker" .)
(Other definitions for makeshift that I've seen before include "What 11 [SEAMSTRESS] might do" , "Provisional" , "Serving as a temporary and less adequate substitute for something" , "Rough and ready" , "not meant to last" .)
(Another definition for stand-in that I've seen is " Substitute".)