Relish one shot, a boundary (9)
I believe the answer is:
shoreline
'a boundary' is the definition.
(shoreline is a kind of boundary)
'relish one shot' is the wordplay.
'shot' is an anagram indicator.
'relish'+'one'='relishone'
'relishone' anagrammed gives 'SHORELINE'.
(Other definitions for shoreline that I've seen before include "Coastal strip" , "Where the sea meets land" , "Land's edge" , "Meeting point of land and sea" .)