Not all of Belloc? Umbrian shows relief (5)
I believe the answer is:
locum
'relief' is the definition.
(locum is a kind of relief)
'not all of belloc? umbrian shows' is the wordplay.
'not all of' indicates the answer is hidden within the clue (I've seen 'not all' mean this).
'LOCUM' is hidden within 'belloc umbrian shows'.
(Other definitions for locum that I've seen before include "temporary occupant" , "Physician's stand-in" , "Professional who stands in for another" , "Stand-in medic" , "Stand-in GP" .)