Minister’s aide’s retreat about key company (6)
I believe the answer is:
deacon
'minister's aide's' is the definition.
(I've seen this in another clue)
'retreat about key company' is the wordplay.
'retreat' becomes 'den' (both can mean a lair or hideout).
'about' means one lot of letters goes inside another.
'key' becomes 'a' (musical key).
'company' becomes 'co' (abbreviation).
'a'+'co'='aco'
'den' placed around 'aco' is 'DEACON'.
(Other definitions for deacon that I've seen before include "Church officer" , "Churchman" , "Cleric - canoed (anag)" , "Cleric ranking just below a priest in Christian churches" , "Church post" .)