One joining military force since getting over shooting expedition (6)
I believe the answer is:
safari
'shooting expedition' is the definition.
'one joining military force since getting over' is the wordplay.
'one' becomes 'i' (Roman numeral).
'joining' is a charade indicator (letters next to each other).
'military force' becomes 'raf' (Royal Air Force).
'since' becomes 'as' (I've seen this in other clues).
'getting over' shows that the letters should be reversed in order.
'raf'+'as'='rafas'
'rafas' written backwards gives 'safar'.
'i' put after 'safar' is 'SAFARI'.
(Other definitions for safari that I've seen before include "A journey to see or hunt wild animals" , "Big game expedition" , "Expedition to observe animals" , "North African capital" , "Hunting expedition (in Africa)" .)