Clue: Property initially unlet, rickety? House lacks one (flat too) (6)
Answer: PENULT
Explanation: Definition: House lacks one (flat too)
**P**(roperty) + *(unlet) 
*"Penult" is a word meaning "the last but one syllable". The words "house" and "flat" only have one syllable, so therefore can't have a penult.