Bottomless sea motif (5)
I believe the answer is:
theme
'motif' is the definition.
(synonyms)
'bottomless sea' is the wordplay.
'bottomless' means to remove the last letter.
'sea' becomes 'themed' (the Med can mean the Mediterranean Sea).
'themed' with its last letter taken away is 'THEME'.
(Other definitions for theme that I've seen before include "argument" , "Pervading idea in art" , "matter for discussion" , "Subject matter, motif" , "idea of the work" .)