March 2nd: class to retire from military service (5)
I believe the answer is:
demob
'retire from military service' is the definition.
(I've seen this before)
'march 2nd class' is the wordplay.
'march' becomes 'demo' (as in a demonstration).
'2nd class' becomes 'B' (as in B-list).
'demo'+'b'='DEMOB'
'to' acts as a link.
(Other definitions for demob that I've seen before include "Return to civvy street" , "Leave the military" , "Release from army" , "Leave the army" , "Discharge from military service" .)