Available for rental (2,3)
'available for rental' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for to let that I've seen before include "Available for renting" , "Estate agent's sign" , "open to offers" , "Available for a tenant" , "Sign of the property market" .)
(Another definition for tolet that I've seen is " for hire".)