Reduced clergyman's department finally (5)
I believe the answer is:
pared
'reduced' is the definition.
(to pare something down is to reduce it)
'clergyman's department finally' is the wordplay.
'clergyman' becomes 'p' (abbreviation for priest).
'department finally' becomes 'ared' (I can't explain this - if you can you should give a lot more credence to this answer).
'p'+'ared'='PARED'
(Other definitions for pared that I've seen before include "Cut bit by bit" , "Peeled the outer surface" , "Of a fruit, peeled" , "Trimmed like pencil." , "Trimmed - peeled" .)