Little, medium or large inland sea (3)
I believe the answer is:
med
'sea' is the definition.
(abbreviation of Mediterranean)
'little medium or large inland' is the wordplay.
I cannot quite see how this works, but
'medium' could be 'm' (as in UK clothing sizes) and 'm' is located in the answer.
The remaining letters 'ed' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for med that I've seen before include "Cote d'Azur sea" , "Sea (informal)" , "Sea (abbr)" , "Mediterranean (colloq.)" .)