Side salad of eel has slice of cucumber on top (7)
I believe the answer is:
chelsea
'side' is the definition.
(I've seen this before)
'salad of eel has slice of cucumber on top' is the wordplay.
'salad of' is an anagram indicator (salad can mean a mixture).
'slice of' indicates taking the first letters (only a small slice of the word).
'on top' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
The initial letter of 'cucumber' is 'c'.
'eel'+'has'='eelhas'
'eelhas' anagrammed gives 'helsea'.
'helsea' put after 'c' is 'CHELSEA'.
(Other definitions for chelsea that I've seen before include "The Blues of Stamford Bridge" , "Hillary Clinton's daughter" , "Barracks here" , "Team in blue" , "Eg, shoplifter" .)