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