Shanty (6)
'shanty' is the definition.
(I know that shanty can be written as shack)
This is all the clue.
(Other definitions for shack that I've seen before include "Hovel" , "Rough dwelling" , "Rough hut or cabin" , "basic accommodation" , "Crudely built hut" .)
(Other definitions for sea that I've seen before include "North, say" , "The Aegean, for example" , "Lesser ocean" , "It's stocked with fish" , "eg Dead, Red" .)
(Other definitions for leanto that I've seen before include "Roof" , "shed" .)