In Sargasso, dash of fizzy water (4)
I believe the answer is:
soda
'fizzy water' is the definition.
(I've seen this before)
'in sargasso dash of' is the wordplay.
'in' says the answer is hidden in the clue.
'SODA' is hidden within 'sargasso dash of'.
(Other definitions for soda that I've seen before include "for use with whisky?" , "Mixer drink" , "Fizzy water, goes well with whiskey" , "Variety of pop" , "Splash served with some spirits" .)