Window art over northern entrances to Science and Oceanography Museum (7)
I believe the answer is:
transom
'window' is the definition.
(I know that transom is a type of window)
'art over northern entrances to science and oceanography museum' is the wordplay.
'over' is a reversal indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'entrances to' says to take the initial letters (I've seen 'entrance to' mean this).
'and' means one lot of letters go next to another.
'oceanography museum' becomes 'om' (this could be a standard abbreviation of which I'm unaware).
The initial letter of 'science' is 's'.
'art' written backwards gives 'tra'.
'tra'+'n'+'s'+'om'='TRANSOM'
(Other definitions for transom that I've seen before include "Cross-beam" , "Bar above window or door" , "Lintel" , "timber for doorframe" , "Horizontal bar across window" .)