Rex needs a break β most unusual (6)
I believe the answer is:
rarest
'most unusual' is the definition.
(I've seen this before)
'rex needs a break' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'needs' says to put letters next to each other.
'break' becomes 'rest' (rest is a kind of break).
'r'+'a'+'rest'='RAREST'
(Other definitions for rarest that I've seen before include "fine to maximum" , "Most scarce or least common" , "least done" , "most excellent" , "Least common, most unusual" .)