Churchman, senior one, holding firm (6)
I believe the answer is:
deacon
'churchman' is the definition.
(I've seen this before)
'senior one holding firm' is the wordplay.
'senior one' becomes 'dean' (I've seen this in another clue).
'holding' indicates putting letters inside.
'firm' becomes 'co' (a firm is a company).
'dean' enclosing 'co' is 'DEACON'.
(Other definitions for deacon that I've seen before include "Churchman" , "Church officer" , "Church official - canoed (anag)" , "Priest's assistant" , "Cleric ranking just below a priest in Christian churches" .)