It's most unusual for religious leader to take a break (6)
I believe the answer is:
rarest
'it's most unusual' is the definition.
(rare can mean unusual or odd)
'religious leader to take a break' is the wordplay.
'leader' says to take the initial letters.
'to take' means one lot of letters go next to another.
'break' becomes 'rest' (a rest is taking a break).
The initial letter of 'religious' is 'r'.
'r'+'a'+'rest'='RAREST'
'for' is the link.
(Other definitions for rarest that I've seen before include "Sartre (anag) -- most uncommon" , "Most scarce or uncommon" , "Most scarce and probably precious" , "Most scarce or infrequent" , "most excellent" .)