Lender almost smiling: it’s on the house! (9)
I believe the answer is:
librarian
'lender' is the definition.
(libraries lend books)
'almost smiling it's on the house' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'smiling' becomes 'riant' (riant means happy or cheerful).
'it's on' says to put letters next to each other (some letters go on the end of others).
'the house' becomes 'libra' (house in the astrological sense).
'riant' with its last letter taken off is 'rian'.
'rian' after 'libra' is 'LIBRARIAN'.
(Other definitions for librarian that I've seen before include "One dealing with books" , "Book keeper" , "Browsers need her" , "lender" , "Custodian of a collection of books" .)