One in the sea grabbed by crazy fool (7)
I believe the answer is:
mislead
'fool' is the definition.
(thesaurus)
'one in the sea grabbed by crazy' is the wordplay.
'one in the sea' becomes 'isle'.
'grabbed by' indicates putting letters inside.
'crazy' becomes 'mad' ('mad' can be a synonym of 'crazy').
'isle' placed within 'mad' is 'MISLEAD'.
(Other definitions for mislead that I've seen before include "Take someone in the wrong direction" , "Hoodwink" , "Cause someone to have a false belief" , "Cause someone to believe something is not true" , "Deceive" .)