Most pious host concealing an untruth (7)
I believe the answer is:
holiest
'most pious' is the definition.
(I've seen this before)
'host concealing an untruth' is the wordplay.
'concealing' is an insertion indicator.
'an untruth' becomes 'lie' (lie is a kind of untruth).
'host' enclosing 'lie' is 'HOLIEST'.
(Other definitions for holiest that I've seen before include "superlatively untouchable" , "Most resembling sacred work" , "most befitting to worship" , "Most pious" , "Most divine" .)