Has first birthday in flat or apartment (10)
I believe the answer is:
maisonette
'apartment' is the definition.
(I know that maisonette is a type of apartment)
'has first birthday in flat' is the wordplay.
I cannot quite see how this works, but
'first' could be 'one' (I've seen this in another clue) and 'one' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'or' is the link.
Can you help me to learn more?
(Other definitions for maisonette that I've seen before include "Two-story flat" , "small property" , "Small house, flat" , "dwelling place" , "Multi-storey apartment" .)