Something unusual, 'I tarry', needs rewriting (6)
I believe the answer is:
rarity
'something unusual' is the definition.
(I've seen this before)
'i tarry needs rewriting' is the wordplay.
'needs rewriting' indicates an anagram (I've seen this in other clues).
'i'+'tarry'='itarry'
'itarry' is an anagram of 'RARITY'.
(Other definitions for rarity that I've seen before include "Unlikely event" , "Something seldom seen" , "Something uncommon" , "something quite unusual" , "Unusual item" .)