See about one small amount of liquid cocktail (6)
'cocktail' is the definition.
(gimlet is a kind of cocktail)
'see about one small amount of liquid' is the wordplay.
'see' becomes 'get' (I've seen this before).
'about' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'small amount of liquid' becomes 'ml' (short for mililitre).
'get' enclosing 'iml' is 'GIMLET'.
(Other definitions for gimlet that I've seen before include "One pierces", "A carpenter's tool", "Tool for boring holes and a cocktail", "Something for boring; cocktail", "Small tool for boring holes".)