Attached shed (6)
'attached shed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lean-to that I've seen before include "Shed, outbuilding" , "Structure whose roof has only one slope" , "Small building resting on side of larger one" , "Outhouse" , "Rough shelter with one-sloped roof" .)
(Other definitions for leanto that I've seen before include "Roof" , "shed" .)