Number given on ad for old-fashioned piece of furniture (10)
I believe the answer is:
fourposter
'ad for old-fashioned piece of furniture' is the definition.
I can't judge whether this defines the answer.
'number given on' is the wordplay.
'number' becomes 'four' (four is a kind of number).
'given' becomes 'poster' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'four'+'poster'='FOURPOSTER'
Can you help me to learn more?
(Another definition for fourposter that I've seen is " sleeping accommodation".)