Give away rental properties before impertinent speech (3,4)
I believe the answer is:
let slip
'give away' is the definition.
(to let a secret slip is to give it away)
'rental properties before impertinent speech' is the wordplay.
'rental properties' becomes 'lets' (properties for let).
'before' means one lot of letters go next to another.
'impertinent speech' becomes 'lip' (lip can mean impertinent or rude speech).
'lets'+'lip'='LET SLIP'
(Other definitions for let slip that I've seen before include "accidentally revealed" , "Inadvertently reveal (something)" , "Fail to keep Secret" , "Allow to fall or reveal a secret accidentally" , "Reveal accidentally, or allow to skid?" .)