Neat reminder devised about a sea (13)
I believe the answer is:
mediterranean
'sea' is the definition.
(Mediterranean is an example)
'neat reminder devised about a' is the wordplay.
'devised' indicates anagramming the letters.
'about' indicates putting letters inside.
'neat'+'reminder'='neatreminder'
'neatreminder' anagrammed gives 'mediterrnean'.
'mediterrnean' placed around 'a' is 'MEDITERRANEAN'.
(Other definitions for mediterranean that I've seen before include "Sea linking three continents" , "Inland sea" , "Sea north of Africa" , "Sea south of Europe" .)