Simply sail off in case of emergency (6)
I believe the answer is:
easily
'simply' is the definition.
(thesaurus)
'sail off in case of emergency' is the wordplay.
'off' is an anagram indicator.
'in' indicates putting letters inside.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'emergency' with its middle taken out is 'ey'.
'sail' with letters rearranged gives 'asil'.
'asil' placed within 'ey' is 'EASILY'.
(Other definitions for easily that I've seen before include "Effortlessly" , "without doubt" , "Without much difficulty" , "With little difficulty" , "without a hitch?" .)