Stand-in student copper discovered bomb outside (5)
I believe the answer is:
locum
'stand-in' is the definition.
(I know that locum is a type of stand-in)
'student copper discovered bomb outside' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'copper' becomes 'Cu' (chemical symbol).
'discovered' indicates the central letters (dis-covered - outer letters removed).
'outside' indicates putting letters inside.
The middle of 'bomb' is 'om'.
'cu' placed into 'om' is 'ocum'.
'l'+'ocum'='LOCUM'
(Other definitions for locum that I've seen before include "One standing in" , "Person filling in" , "Stand-in physician" , "Professional who substitutes temporarily for another" , "A substitute or stand-in doctor" .)