Venture to say he left house unsafe (9)
I believe the answer is:
hazardous
'unsafe' is the definition.
(similar in meaning)
'venture to say he left' is the wordplay.
'venture' becomes 'hazard' (both can mean to offer or suggest).
'to say he left' becomes 'ous' (I can't justify this - if you can you should give a lot more credence to this answer).
'hazard'+'ous'='HAZARDOUS'
'house' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for hazardous that I've seen before include "Perilous" , "Carrying risk, dangerous" , "Unsafe" , "Very risky, dangerous" , "Involving risk or danger" .)