Last resort for Jack (4)
I believe the answer is:
salt
'jack' is the definition.
(thesaurus)
'last resort' is the wordplay.
'resort' indicates an anagram.
'last' anagrammed gives 'SALT'.
'for' is the link.
(Other definitions for salt that I've seen before include "Sea dog" , "Preservative" , "Crystalline substance used to season food" , "A defence against ice" , "Common substance" .)