Sub rolling pass? I'm not sure (5)
I believe the answer is:
locum
'sub' is the definition.
(locum can mean a substitute)
'rolling pass? i'm not sure' is the wordplay.
'rolling' says the letters should be written backwards.
'pass?' becomes 'col' (col is a term for a mountain pass).
'i'm not sure' becomes 'um' (expression of hesitation).
'col' in reverse letter order is 'loc'.
'loc'+'um'='LOCUM'
(Other definitions for locum that I've seen before include "Stand-in medic" , "Medical temp?" , "Professional temp" , "Someone standing in temporarily for another of the same profession" , "Person filling in" .)