Essentially block alumna from working as temporary stand-in (5)
I believe the answer is:
locum
'temporary stand-in' is the definition.
'locum' can be an answer for 'stand-in' (locum is a kind of stand-in). I'm not certain of the 'temporary' bit.
'essentially block alumna from working' is the wordplay.
'essentially' says to take the centre.
'alumna from working' becomes 'um' (I can't justify this - if you can you should give a lot more credence to this answer).
The middle of 'block' is 'loc'.
'loc'+'um'='LOCUM'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for locum that I've seen before include "Doctor's temporary replacement" , "Stand-in GP" , "Professional who substitutes temporarily for another" , "Stand-in professional" , "Person filling in" .)