Ensure boat lashed in seaside resort (10)
I believe the answer is:
eastbourne
'seaside resort' is the definition.
(English resort town)
'ensure boat lashed' is the wordplay.
'lashed' is an anagram indicator.
'ensure'+'boat'='ensureboat'
'ensureboat' anagrammed gives 'EASTBOURNE'.
'in' acts as a link.
(Other definitions for eastbourne that I've seen before include "Resort close to Beachy Head" , "somewhere on the south coast" , "popular retirement venue" , "Channel resort" , "Resort in East Sussex" .)