Fundamental crack lets in sea (9)
I believe the answer is:
essential
'fundamental' is the definition.
(thesaurus)
'crack lets in sea' is the wordplay.
'crack' indicates an anagram.
'lets'+'in'+'sea'='letsinsea'
'letsinsea' anagrammed gives 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "Type of oil" , "Key" , "Vital" , "absolutely neccessary?" , "Indispensible" .)