Church Mass arranged in heart of Kent castle for one (8)
I believe the answer is:
chessman
'castle for one' is the definition.
(castle is a kind of chessman)
'church mass arranged in heart of kent' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'arranged' indicates an anagram.
'in' means one lot of letters goes inside another.
'heart of' means to look at the middle letters.
The centre of 'kent' is 'en'.
'mass' is an anagram of 'ssma'.
'ssma' going within 'en' is 'essman'.
'ch'+'essman'='CHESSMAN'
(Other definitions for chessman that I've seen before include "Eg, bishop, knight" , "Playing piece" , "that's played on board" , "Eg, bishop or rook" , "queen, perhaps" .)