Posh churchgoer protected by solitary item of tack (9)
I believe the answer is:
surcingle
'item of tack' is the definition.
(part of a horse's tack)
'posh churchgoer protected by solitary' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'churchgoer' becomes 'rc' (abbreviation for Roman Catholic).
'protected by' indicates putting letters inside.
'solitary' becomes 'single' (similar in meaning).
'u'+'rc'='urc'
'urc' inserted within 'single' is 'SURCINGLE'.
(Another definition for surcingle that I've seen is " Equestrian band".)