Among party, soldiers occupying building (6)
I believe the answer is:
indoor
'occupying building' is the definition.
(inside a building)
'among party soldiers' is the wordplay.
'among' becomes 'in' (eg among or in a group).
'party' becomes 'do' (do is a kind of party).
'soldiers' becomes 'or' (Other Ranks).
'in'+'do'+'or'='INDOOR'
(Other definitions for indoor that I've seen before include "(Used) within a building" , "Taking place within a building" , "Not outdoor" , "Situated under cover" , "in chambers?" .)