Seaplane crashed across end of Southwold promenade (9)
I believe the answer is:
esplanade
'promenade' is the definition.
(I know that promenade can be written as esplanade)
'seaplane crashed across end of southwold' is the wordplay.
'crashed' indicates an anagram.
'across' indicates putting letters inside.
'end of' says to take the final letters.
The last letter of 'southwold' is 'd'.
'seaplane' with letters rearranged gives 'esplanae'.
'esplanae' going around 'd' is 'ESPLANADE'.
(Other definitions for esplanade that I've seen before include "Seaside road" , "Open, level space for walks or drives" , "Promenade" , "Long wide pavement next to a beach" , "prom" .)