Slug in bottom of glass with syrup (4)
I believe the answer is:
swig
'slug' is the definition.
('swig' can be a synonym of 'slug')
'bottom of glass with syrup' is the wordplay.
'bottom of' says to take the final letters.
'with' means one lot of letters go next to another.
'syrup' becomes 'wig' ('syrup of fig' is Cockney rhyming slang for 'wig').
The last letter of 'glass' is 's'.
's'+'wig'='SWIG'
'in' is the link.
(Other definitions for swig that I've seen before include "Large gulp from the bottle" , "Drink quickly from bottle" , "Drink (a deep draught)" , "Large draught of drink" , "(Take) quick drink from bottle" .)