Shelter under roof of houseboat in port (7)
I believe the answer is:
harbour
'shelter' is the definition.
(harbouring is a kind of sheltering)
'under roof of houseboat in port' is the wordplay.
'under' means one lot of letters go next to another.
'roof of' suggests taking the first letters (in a down clue, the top letter).
'in port' becomes 'arbour' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The first letter of 'houseboat' is 'h'.
'h' next to 'arbour' is 'HARBOUR'.
Can you help me to learn more?
(Other definitions for harbour that I've seen before include "Safe mooring for ships" , "Ship haven or shelter" , "Conceal" , "Place of refuge" , "Coastal haven" .)