Property agreement invalid as I'm three! (9)
I believe the answer is:
timeshare
'property agreement' is the definition.
'invalid as i'm three' is the wordplay.
'invalid' is an anagram indicator.
'as'+'im'+'three'='asimthree'
'asimthree' with letters rearranged gives 'TIMESHARE'.
(Other definitions for timeshare that I've seen before include "Multiple property ownership" , "flat, perhaps" , "holiday location" , "Holiday arrangement" , "temporary accommodation" .)