Bitter syrup produced by Missouri girls (8)
I believe the answer is:
molasses
'bitter syrup' is the definition.
'by missouri girls' is the wordplay.
'by' says to put letters next to each other.
'missouri' becomes 'mo'.
'girls' becomes 'lasses' (lass is a kind of girl).
'mo'+'lasses' is 'MOLASSES'.
'produced' acts as a link.
(Other definitions for molasses that I've seen before include "Thick treacle left over after refining raw sugar" , "Black treacle" , "Dark syrup from raw sugar" , "Syrup drained from raw sugar for some lass" , "what refinement can bring" .)