Simple structure (3)
(Other definitions for leanto that I've seen before include "Roof" , "shed" .)
'simple structure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hut that I've seen before include "Small crude dwelling" , "Small crudely-built shelter" , "Simple building" , "'Shack, hovel (3)'" , "Simple house or shelter" .)