Sort of average staff holding back help (6)
I believe the answer is:
median
'sort of average' is the definition.
'staff holding back help' is the wordplay.
'staff' becomes 'man' (as in staffing or manning a particular place).
'holding' is an insertion indicator.
'back help' becomes 'edi' (I can't justify this - if you can you should believe this answer much more).
'man' going around 'edi' is 'MEDIAN'.
Can you help me to learn more?
(Other definitions for median that I've seen before include "statistical quantity" , "Situated or passing through the middle" , "In the middle; statistical value" , "Central reservation on a road" , "Passing through the middle of Medina" .)