Senior cleric taking care of a junior one (6)
I believe the answer is:
deacon
'a junior one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'senior cleric taking care of' is the wordplay.
'senior cleric' becomes 'dean'.
'taking' indicates putting letters inside.
'care of' becomes 'co' (c/o can mean 'care of' when addressing mail).
'dean' going around 'co' is 'DEACON'.
(Other definitions for deacon that I've seen before include "A member of the lower orders of clergy" , "An ordained minister" , "Cleric ranking just below a priest in Christian churches" , "Churchman" , "Junior minister" .)