A spruce shelters posh courtyard (6)
I believe the answer is:
atrium
'courtyard' is the definition.
(atrium is a kind of courtyard)
'a spruce shelters posh' is the wordplay.
'spruce' becomes 'trim' (synonyms).
'shelters' means one lot of letters goes inside another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'trim' placed around 'u' is 'trium'.
'a'+'trium'='ATRIUM'
(Other definitions for atrium that I've seen before include "lobby" , "Central hall or court" , "Central court of old Roman house, or heart cavity" , "High central hall in large building" , "Central court of ancient Roman house" .)