Stand-in minister starts to organise choir in Mass? Not all of it (5)
I believe the answer is:
locum
'stand-in minister' is the definition.
(church minister standing in for another)
'starts to organise choir in mass? not all of it' is the wordplay.
'starts to' says to take the initial letters.
'in' is an insertion indicator.
'mass?' becomes 'lump' (I have seen 'Shapeless mass' mean 'lump' so perhaps 'mass' could also mean 'lump').
'not all of it' means to remove the last letter.
The initial letters of 'organise choir' is 'oc'.
'lump' with its last letter taken away is 'lum'.
'oc' put into 'lum' is 'LOCUM'.
(Other definitions for locum that I've seen before include "Professional who substitutes temporarily for another" , "Temporary deputy for doctor" , "Doctor's deputy" , "Doctor's temporary replacement" , "Physician's stand-in" .)