Body of water of same dimensions (3)
I believe the answer is:
med
'body of water' is the definition.
(short for Mediterranean)
'of same dimensions' is the wordplay.
'of' indicates the answer is hidden within the clue.
'MED' can be found hidden inside 'same dimensions'.
(Other definitions for med that I've seen before include "Sea (abbr)" , "Cote d'Azur sea" , "Sea (informal)" , "Mediterranean (colloq.)" .)