Stand-in starts to learn off Cambridge University master (5)
I believe the answer is:
locum
'stand-in' is the definition.
(locum is a kind of stand-in)
'starts to learn off cambridge university master' is the wordplay.
'starts to' indicates taking the first letters.
'cambridge university' becomes 'cu'.
'master' becomes 'm'.
The first letters of 'learn off' is 'lo'.
'lo'+'cu'+'m'='LOCUM'
(Other definitions for locum that I've seen before include "Professional temp" , "Physician's stand-in" , "Stand-in physician" , "Medical temp?" , "Temporary deputy for doctor" .)