The sides contain clear cocktail syrup (7)
I believe the answer is:
treacle
'syrup' is the definition.
(I know that treacle is a type of syrup)
'the sides contain clear cocktail' is the wordplay.
'sides' suggests removing the centre (only the letters on the sides are used).
'contain' means one lot of letters goes inside another.
'cocktail' indicates anagramming the letters.
'the' with its middle taken out is 'te'.
'clear' is an anagram of 'reacl'.
'te' going around 'reacl' is 'TREACLE'.
(Other definitions for treacle that I've seen before include "Something sticky" , "Thick syrup from sugar cane" , "Something cloyingly sweet" , "Sticky fluid" , "flattery" .)