Impenetrable island shelters right criminal (8)
I believe the answer is:
airtight
'impenetrable' is the definition.
(I've seen this before)
'island shelters right criminal' is the wordplay.
'island' becomes 'ait' (I've seen this before).
'shelters' means one lot of letters goes inside another.
'criminal' indicates an anagram.
'right' anagrammed gives 'rtigh'.
'ait' placed around 'rtigh' is 'AIRTIGHT'.
(Other definitions for airtight that I've seen before include "Sure" , "Without weak points" , "Firmly closed" , "Not allowing gas to enter or escape" , "(Of a defence) having no weaknesses" .)