Mean to turn to drink a lot: it’s on the house! (7)
I believe the answer is:
rooftop
'it's on the house' is the definition.
(a roof on top of a house)
'mean to turn to drink a lot' is the wordplay.
'mean' becomes 'poor' (poor can mean meagre or mean).
'to turn' shows that the letters should be reversed in order.
'to drink' means one lot of letters goes inside another.
'a lot' becomes 'oft' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'poor' back-to-front is 'roop'.
'roop' placed around 'oft' is 'ROOFTOP'.
Can you help me to learn more?
(Other definitions for rooftop that I've seen before include "Of Porto (anag.)" , "I could be slated" , "Elevated point" , "Upper surface of building" , "Building's upper surface" .)