Is due to cracks in the superficies (7)
I believe the answer is:
outside
'the superficies' is the definition.
(thesaurus)
'is due to cracks' is the wordplay.
'cracks' indicates anagramming the letters (I've seen 'cracking' mean this).
'is'+'due'+'to'='isdueto'
'isdueto' is an anagram of 'OUTSIDE'.
'in' is the link.
(Other definitions for outside that I've seen before include "having emerged from the tunnel?" , "Where smokers now often must go" , "Alfresco" , "Without" , "External surface" .)