Record-keeper about to get essential stuff, unusual mostly (9)
I believe the answer is:
registrar
'record-keeper' is the definition.
(I know that record-keeper can be written as registrar)
'about to get essential stuff unusual mostly' is the wordplay.
'about to get' becomes 're' (I've seen this in other clues).
'essential stuff' becomes 'gist' (the gist is of something is its essence).
'unusual' becomes 'rare' (rare can mean unusual or odd).
'mostly' means to remove the last letter.
'rare' with its last letter taken off is 'rar'.
're'+'gist'+'rar'='REGISTRAR'
(Other definitions for registrar that I've seen before include "clerk" , "Mid-ranking doctor" , "Record keeper - hospital doctor" , "Official record keeper" , "certifies births and deaths" .)