Retire after serving protest by bishop (5)
I believe the answer is:
demob
'retire after serving' is the definition.
(demobilise - removing soldiers from service)
'protest by bishop' is the wordplay.
'protest' becomes 'demo' (I've seen this before).
'by' means one lot of letters go next to another.
'bishop' becomes 'b' (abbreviation used in chess).
'demo'+'b'='DEMOB'
(Other definitions for demob that I've seen before include "put out of service" , "Discharge from forces" , "Take out of action" , "Release from armed forces" , "remove from service" .)