Not exactly La Mer, nevertheless the sea! (3)
I believe the answer is:
med
'the sea' is the definition.
(short for Mediterranean)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for med that I've seen before include "Sea (informal)" , "Sea (abbr)" , "Cote d'Azur sea" , "Mediterranean (colloq.)" .)