Worker finally has a break, which is most unusual (6)
I believe the answer is:
rarest
'is most unusual' is the definition.
(rare can mean unusual or odd)
'worker finally has a break' is the wordplay.
'finally' says to take the final letters.
'has' is a charade indicator (letters next to each other).
'break' becomes 'rest' (a rest is taking a break).
The final letter of 'worker' is 'r'.
'r'+'a'+'rest'='RAREST'
'which' is the link.
(Other definitions for rarest that I've seen before include "least done" , "Least common, most unusual" , "fine to maximum" , "Sartre (anag) -- most uncommon" , "Most scarce and probably precious" .)