Paradise island showing up lack of volcanic activity? (6)
I believe the answer is:
avalon
'paradise island' is the definition.
(island in Arthurian myth)
'up lack of volcanic activity?' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'lack of' becomes 'no' (lack of money is no money).
'volcanic activity?' becomes 'lava' (lava flows from a volcano).
'no'+'lava'='nolava'
'nolava' backwards is 'AVALON'.
'showing' acts as a link.
(Other definitions for avalon that I've seen before include "Island where King Arthur's body was buried" , "Fabled Arthurian isle" , "Isle of the Blessed in Arthurian legend" , "Where King Arthur's body was brought" , "Mythical isle" .)