I tarry in resort: that’s not seen often (6)
I believe the answer is:
rarity
'that's not seen often' is the definition.
'i tarry in resort' is the wordplay.
'in resort' indicates an anagram.
'i'+'tarry'='itarry'
'itarry' with letters rearranged gives 'RARITY'.
(Other definitions for rarity that I've seen before include "Something seldom seen" , "Find" , "something quite unusual" , "Infrequent occurrence" , "Uncommon item or event" .)