Look, I'm not sure about coming in as substitute (5)
I believe the answer is:
'substitute' is the definition.
(locum can mean a substitute)
'look i'm not sure about coming in' is the wordplay.
'look' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'i'm not sure' becomes 'um' (expression of hesitation).
'about' becomes 'c' (abbreviation for 'circa').
'coming in' means one lot of letters goes inside another.
'loum' going around 'c' is 'LOCUM'.
'as' is the link.
(Other definitions for locum that I've seen before include "Stand-in, for a doctor say" , "temporary occupant" , "One standing in" , "Physician's stand-in" , "Relief doctor" .)