Fanatic to survive bracing term at sea (9)
I believe the answer is:
extremist
'fanatic' is the definition.
(thesaurus)
'survive bracing term at sea' is the wordplay.
'survive' becomes 'exist' ('exist' can be a synonym of 'survive').
'bracing' indicates putting letters inside (bracing can mean securing or surrounding).
'at sea' is an anagram indicator.
'term' anagrammed gives 'trem'.
'exist' enclosing 'trem' is 'EXTREMIST'.
'to' is the link.
(Other definitions for extremist that I've seen before include "Views far beyond the norm" , "His views are anything but neutral" , "One far from moderate" , "Ultra" , "One with highly non-standard views" .)