A holy man grows old by degrees (6)
I believe the answer is:
stages
'degrees' is the definition.
(I know this)
'a holy man grows old by' is the wordplay.
'a holy man' becomes 'st' (abbreviation for saint).
'grows old' becomes 'ages' ('age' can be a synonym of 'grow old').
'by' means one lot of letters go next to another.
'st'+'ages'='STAGES'
(Other definitions for stages that I've seen before include "legs" , "Platforms in theatres" , "Fixes" , "Theatrical platforms or portions of a journey" , "Coaches" .)