A Republican in lie? Most unusual (6)
I believe the answer is:
rarest
'most unusual' is the definition.
(I've seen this before)
'a republican in lie?' is the wordplay.
'republican' becomes 'r' (abbreviation).
'in' is an insertion indicator.
'lie?' becomes 'rest' (liing is a kind of resting**).
'a'+'r'='ar'
'ar' going inside 'rest' is 'RAREST'.
(Other definitions for rarest that I've seen before include "Most scarce or uncommon" , "least done" , "fine to maximum" , "Least common, most unusual" , "Most scarce or least common" .)